The scrap metal glitch explained in detail with a quick start guide.
I have been a bit more thorough.  Some people like thorough.

To perform this glitch you will need the following:
6 ScrapMetal
2 Stealth boys OR high sneak (luck doesn’t hurt either).  Stealth boys still recommended.

The Quick Guide

This will take you about 10-20 minutes to accomplish, but it is well worth it (at least if you want to speed level to 20 or 30)
The quick guide is as follows:
1. Sell 6 scrap metal to Protector Casdin (2 at a time/Fort Independence).  Your Reward Doesn’t Matter (except grenades as they add weight, but they won’t affect the glitch).
2. Stealth boy.
3. Save.
4. Steal (pickpocket) 6 scrap metal (all of it).  If you fail here Reload the save.
5. Sell (or drop) 4 scrap metal.
6. Save (your stealthboy should still be active).
7. Try and swipe his rifle.
8. Sell him scrap metal until he has about 25 Percent more than you want.
9. Wait for 73 hours.
10. Fast travel SOMEWHERE ELSE.  Then back.
11. Stealth boy.
12. Save.
13. Steal most of the scrap back.  Reload save if necessary.  Stealing it back a chunk at a time (IE 50 at a time) will not work.  It will glitch on the “last” chunk you stole, even if you didn’t exit the pickpocket screen. Success depends on luck as well, and at a luck of 5, I find that if I hit the Left trigger once when selecting how much scrap to steal, I am successful (approx 150ish). So try to steal back approx 75 percent of what you sold him, if it’s over 100.  I did 250 and stole back 200 successfully.  If you can get your luck up this will help.
14. Save.
15.  Drop all but 2 pieces of scrap metal
16. Steal his rifle. You will get caught.
18. Never pick up scrap metal again.
If done properly you should have an invisible scrap metal in your inventory equal to what you were able to steal minus 2. You can level up with Walter in about 10 minutes with the take all my scrap metal dialog option. I went from 10 to 30 and got about 119000 caps as well, so not bad.

The Complete Glitch Explained (as best as I can figure)

Now starts the in depth part of the guide with technical descriptions (for those of you who are curious or simply want to understand the glitch).

1. Sell 6 scrap metal to Protector Casdin (2 at a time/Fort Independence).  Your Reward Doesn’t Matter (except grenades as they add weight, but they won’t affect the glitch).  6 is the minimum but any even number above 6 will work.  I tried it with 4 and the game said no (PC Version).  6 is recommended for step 1 as its quick and easy.  After you sell it to Casdin make sure you have none in your inventory.  At this point the game assigns ownership of the scrap metal to Casdin.

2. Stealth Boy.  Ups your sneak to 100 and makes a cloak field, which makes it a hell of a lot easier to steal back your scrap metal.

3. Save. If you fail pickpocketing you will need to reload your save.  I also save after popping the stealthboy so I don’t have to re-pop it if I fail the steal and need to reload.

4. Steal (pickpocket) all the scrap metal he has on him.  If you fail here Reload the save.  At this point the scrap-metal’s ownership is assigned to Casdin, and the game keeps track of how much you stole from him (6).

5. Sell or drop 4 scrap metal (or all of it except for 2 pieces).  You need to keep 2 in your inventory.  Remember the game sells in groups of 2 to Casdin.  If you’re stingy, sell. If you’re lazy, drop.

6. Save (your stealthboy should still be active.  You will know because if you bring up your pipboy you can see through most of it).

7. Get caught stealing. When you get caught stealing the game causes the character to take back what you stole.  However, it takes back the original amount stolen (so long as you kept at least 2, it seems that if you only have 1, it just says “Scrap metal removed” instead of “### Pieces of Scrap Metal Removed”.  So it takes back the 6 scrapmetal you stole and subtracts it from what you have in your inventory (2).  So the game subtracts 6 scrap metal instead of 2 leaving you with -4 scrap metal.  The game is not equipped to handle this negative, non-zero value, so it treats it like a positive value (+4) but since it’s negative, it doesn’t put the scrap metal in your inventory (it’s invisible and weightless), but when you come across someone who wants your scrap metal (Casdin, Walter, that ghoul in underworld, the ammo press if you have the pit add-on, etc) the game uses that value.  The reason it works well, is because the game takes that value, and subtracts it from itself when you use scrap metal for something, and it still has a negative value, and therefore leaves the number alone.  The value is permanent if you never pick up another piece of scrap metal again.  I theorize that the game uses a seperate value dedicated to the amount of scrap metal you have, and it rely’s on this number before it does anything like remove it from your inventory. 

8. Sell him scrap metal until he has more than 200 (202, 204… etc) or approx 33% more than you want (If you want 150 glitched, sell him 200)  as 150 is 75% of 200, and 200 is 133% of 150.  We are using the scrap metal glitch to create more scrap metal to use so we can do the scrap metal glitch with more scrap.  Cleaver huh?  I find that you will be much more successful in stealing back approx 75% of the scrap metal that he has in his inventory.  The best I was able to get was 250 in his inventory and I was able to steal 80 percent (after many fails) or about 200 leaving me with a 198 glitched scrap metal in my inventory.  The second time I did this I went to 200 but could not successfully steal more than 150 (at 1 time).  This must be done in 1 go (you cannot do it multiple times, even if you don’t exit the pickpocket screen, as the game will use the last amount stolen for the glitch.  Example: You steal 202 scrap in chunks of 50, 50, 50, and 52.  The game will glitch on the 52 scrap you stole last).

9. Wait for 73 hours. The reason for this is the game resets a lot of variables every 72 hours.  Start laying waste to the citizens of Rivet City? Guards all up in your business? Wait 72 hours and its “Good to see ya.”  These people have the attention span of a ham sandwich.  Anyways if you get caught pickpocketing Casdin 2 times in a 72 hour period he gets all “That’s not yours” and “I’m gonna murder me a mothaf…”, well, all butt-hurt anyways.  He’s very difficult to sell to when he’s trying to kill you.  Wait 72 hours and he forgets you stole his scrap metal and helped him look for it like a tweaker. I add an hour to be thorough. but in theory it should be fine.

10. Fast travel somewhere else.  If you do not leave the area it will continue to tell you that Casdin already caught you pickpocketing him.  Fast traveling from Fort Independence to Fort Independence will not work.  I just went to Megaton and then Immediatelly back.  You will probably need to do this to keep track of the values in step 8 (I just counted each time I saw 5.56 on the screen until I hit 100, remember it sells in units of 2).

From here on out you are basically repeating the glitch.
11. Stealth Boy. It has worn off by this point.
12. Save.

13. Steal back that 75% we talked about.  Reload save if necessary. Stealing chunks at a time will not work, it must be in 1 go. The more weight you steal the higher your odds of getting caught, but this seem’s to be offset by the total amount of weight.

14. Save.
15.  Drop all but 2 pieces of scrap.  I figured out that you do not need to sell it back to him like so many other guides say, as long as you have more than 1 stolen scrap in your inventory the game will “remove” what you stole leaving you with the negative number. If you keep only 1 piece of scrap it will simply say “Scrap metal removed” instead of “### pieces of scrap metal removed” and it won’t glitch.
16. Save.
17. Try and swipe his rifle.
18. Never pick up scrap metal again.
Go see Walter.  Level up.  Repeatedly.

Additional Notes: 
1. I have tried to drop 1 piece of scrap metal (so that i only had 1 in the inventory) so that way when the number got removed, it would be negative an extra number (ie it would remove 6 and 4 from 1 leaving a negative 5 or 5 scrap metal (the first value, the second value, after dropping 1) but it did not work.
2. This guide is nice because you don’t need a high investment to get a high return.
3. The Stealth Boys are optional (But damned convenient and you will reload less).
4. Please don’t copy this, just link to it.  This way my server will pay for itself (hopefully… one day).