Intrestingly, you can backstab w/ a quarterstaff.

To get invisible again after you have backstabbed you (the theif backstabing) will need to either get out of visual range of (I believe ALL) monsters in the area and then try your stealth ability again, or stay where you are and turn invisible via a ring or quaffing a potion then go for another backstab.

Your Stealh/invisiblity can be removed via spells - the most powerfull being True S.. (TOB expansion has a cloak somewhere in the game that allow your char. to become undetectable via these spells).

I have tried and it is just a normal attack. Which kind of sucks but I guess it keeps the game in balance.