PetrusOctavianus: Interesting...could you try it on some large (more than one square) monsters and see if it works on them?
OK, this time instead of heading for the slums I instead took the boat to Sokol Keep. I entered and head west towards the scorpion room.
This fight was a bit more tricky to determine if backstab was working due to: (1) my thieves missing a lot of the time because of the scorpion's low AC; and (2) the scorpions mowing my thieves down when they attacked.
However, after many many attempts I conclusively determined that YES, backstab is still working correctly against them, whether I attack diagonally, longwise, or shortwise across them.
I found the relevant section in the rulebook (below):
In a combat, the first and second attackers strike at the defender's front. The third attacker strikes at the defender's rear, unless all the attackers are adjacent. The fourth and any additional attackers strike at the defender's rear. The defender's AC is substantially reduced against rear attacks.
A thief forms the only exception to the automatic facing rules. If the thief attacks from exactly opposite the first attacker, he can backstab. A backstab has a better chance of hitting the defender, and does additional damage when it does hit.
So, it looks like if it requires 2 setups in the PC version of PoR before you can backstab, then the "exception" for the thieves is not being applied properly.
Also, just for fun, I tried ganging up on a scorpion with my thieves indiscriminately (just trying to get the 3rd and 4th and subsequent attacks in) WITHOUT trying to setup a backstab, and this was not successful, the 3rd and subsequent attacks were not automatic backstabs.