I was reading a reddit post earlier, and someone was saying that rage actually currently stacks with backstab, so I thought maybe they reworked how "backstab" is defined, maybe instead of 100% crit, it's now "x2 damage", sort of like how sneak attack works in skyrim, gets a damage multiplier but not actually crits.
I feels like this approach is better than the plain backstab = 100% crit, as rogues naturally gets crit chance from wits and other sources, and the crit chance would be useless if backstab functions in such a way.
I cannot confirm this at the moment, but others may feel free to try it when they have free time.
I just tested as a sanity check, and I can confirm this is 100% false. You cannot also crit on a Backstab.
I agree that the system you describe would be better. I'd love it if they changed it to something like that.
Edit: The following link details my comprehensive tests showing that Backstab and Crit do not stack:
http://larian.com/forums/ubbthreads.php?ubb=showflat&Number=593437#Post593437