Anyone found out whether it's possible with Katanas/Scimitars etc yet?

You can backstab with both katanas and scimitars, as well as longswords, shortswords and daggers.

Very strange my assassin somehow wasn't able to backstab but suddenly when i switcht from short to long swords he did
and now all you see is quadruble damage
16*5=80 or something very very cool
i never knew you could do it with undead to

Can't use a two-handed sword, it seems. Pity.

You get a message about it being an "inappropriate weapon" for backstabbing.

Try to imagine a fighter/thief lugging a two handed sword around while invisible before attempting to shove it in someone's back and you'll see why.

Halscion, 2H swords are usually about 6 feet long, to give you a bit of perspective.

hehe, well why not call it backhacking then?
Though I think that spears would be excellent to backstab with! I mean they´re made to stab with and they don´t have to be that bloody long...

It's a bit awkward to backstab with a pole arm. An example scenario would be an assassin, his trusty shortsword at the ready, sneaking through a mansion's dark hallways towards his target: its wealthy benefactor who happens to be writing his memoires in his bedroom. Firstly, the idea of a thief 'sneaking' around with a spear at all seems a little ludicrous, and then the thought of him hefting the thing before impaling the miserly old bastard on it is a bit strange, too. Personally, I don't even think longswords should be a viable backstabbing weapon.

Aha, well you definately should read some Robert Jordan, and I didn´t mean polearms in general, only spears.

I suppose somewhere in his writing there's an exotic, well-known assassin whose weapon of choice is the spear... but I'd wager it's not really AD&D-esque. Spears are usually classified as pole arms - they're essentially a weapon on the end of a pole.