You can backstab with longswords, Katanas and some others too. Personally I'd rather you could only B/S with daggers or at the outside a short sword. I think the game lets itself down in this department, particularly as Katanas are in reality two handed swords. Imagine a Halfling Backstabbing with a weapon that's nearly as tall as he/she is, it's ridiculous.........still, there ya go.
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It's bleedin dark down here, slippery too.

Man, that's a pisser.