Now heres a bonehead question but how do I make my thief backstab somebody, I've been through the game twice now(no cheating) and haven't figured it out yet.

Behind the link is some information about how backstabbing works in BG-1 and BG-2 :D at least I didn't notice any major difference compared to IWD ;) except one thing:

-Unlike BG-1/ 2 hide in shadows doesn't work instanteiously, you have to move into shadow and ou're only hidden when you finally succeeded.
http://www.sorcerers.net/Games/BG2/thieves.htm

One of the walkthroughs tells you exactly how to do it. I think you also have to literally walk up behind them, and be hidden. There's some modifier or equipped item which negates this requirement, but it's been a long time since I really read through the walkthrough.

Actually, there is one other major difference that I've found with the thieves hide ability.

Unlike the other IE games, when your "move silently" skill is high enough, you can dissapear in plain sight of the enemy.