I remember on Fallout 3 there at the beginning of the game when you got your pip-boy if you say you hate it and how do you get it off he says that you cant something about it binding itself to your skin and he says "I could tell you some stories about taking them off the dead old folks."And at the beginning of Fallout New Vegas Doc Mitchel gives you his.So how did he take it off?

Actually, It was stated later in Fallout 3 that it was NOT impossible to get off, You just need a good technician or someone good with mechanics.

Or with the biometrics needed to remove it. Its lock is set to some manner of biometric signature on the wearer, like maybe a thumbprint or DNA scan or something.

I dont doubt you but where did you hear that?

Im not sure if this is what other say but i remember (mind you im not to sure) but in the begining of fallout 3 when you first get your pip boy the guy who gives it to you say,

say you need to work hard at them like the pip boy is binded to the persons arm

Well, Doc Mitchell does wear long sleeves, so maybe his left forearm doesn't look so good.

The guy who gives it to you and your dad I think.

I've actually forgotten, But it's in the game.

It's been a while, but the likely character in FO3 who would have the knowledge/expertise to make a comment like that would be Stanley.

Here is what the Wiki says:
http://fallout.wikia.com/wiki/Pip-Boy#Pip-Boy_3000

Wow. I thought this was just a really ribald joke.................

Yeah hes the one who says you cant take it off.