I have a multiclass fighter/thief and I was wondering if you can backstab with any weapon -- it'd be nice to backstab with my trusty Spider's bane as opposed to my cute butter knife :P

Why don't you use the Short Sword of Backstabbing?

A note on backstabbing:

You can use any weapon, BUT: in TOTSC, you need to be BEHIND your possible victim and there must be shadows (or night) that you can hide in. You can tell a successful backstab by the amount of damage done. (There is no feedback saying 'backstab successful').

Hmm...would casting "Invisibility" on the thief aid his backstabbing?
One other question, is it true that in BG2 strength modifiers(eg having Str(92) will +5 to damage) won't be included in calculating the backstab damage? It seems pretty unnecessary since the game developers can simply take out the excess potions of Strength instead.

1, Invisibilty doesn't work, you'll have to hide...

2, The problem is that the backstab applies to the thief only, and to the thief part of combined characters. Thieves, however, don't have 18+ modifiers.

To clarify my question about the Str(92) thief, the fella is a dual-classed fighter/thief, so by right, he can legitimately 'inherit' the Str of his former class. I'm still unclear whether strength will have an impact on BG2 though, since the developers mentioned (on a certain website) that the backstab may become too powerful due to the surplus of strength potions.
I've tried backstabbing with the Spider's Bane, but it doesn't work -- "wrong wpn for backstabbing"; straightforward hacking is okay though.

mir_ror: that's just what I said: ONLY THE THIEF HALF..... (for exactly those reasons)

While a multiclassed Fighter/Thief can use any weapon that a Fighter can, only the Thief weapons are suitable for backstabbing. In BG, exceptional strength does get multiplied for backstabbing. I once had a dual classed Fighter/Thief backstab for 118 pts of damage!! (a 20 was rolled) Too bad it was wasted on a common creature. I've heard in BG2 that strength will NOT be multiplied. Also, the +3 Short Sword of Backstabbing may not be the best weapon out there for the task.........

You cannot use any weapon to backstab with. IMHO Varscona (Long Sword +2)is the best piece to do this. You might even try Twinkle :) Two-handed swords won't work, it's too big. You'll get the following message:

Weapon not suited for backstab (or something like that).

PS: you can also use Improved Invisibility for this...

Petrarcus, you found improved invisibility in BG? Where?