Everything You Wanted to Know About Backstabs but were Afraid to Ask. <- wish somebody would write the book
Ok, this is what I know from experimentation -
ALL bladed weapons that you can hold in ONE hand can be used, EXCEPT axes and bastard swords. Axes are for Dwarfs. Bastard Swords are for... um... well...
As far as I know, you cannot use a staff to backstab because a) it is a 2 handed weapon, and b) it has no blade. I've tried it with the Staff of the Magi. Don't remember trying with any other staff. I'd say that if it works it's probably a bug. SHH! Thieves need all the help they can get, and there's a Code of Silence, remember?