Having just asked a similar question myself, I'll share what I have learned so far. Here's the link to the thread also:
viewtopic.php?t=36072&highlight=
Bioware broke the Diplomacy skill into two parts: Persuade and Taunt. For use IG and practically speaking it is persuade skill check vs. persuade skill check. (Persuade skill check = 1d20 + Persuade Ranks + Wisdom modifier.) If there is a tie, the person with the highest modifer "wins". If the modifiers are the same, reroll.
Concerning Intimidate: 3.5 rules state the opposed check is a modified character check which is 1d20 + char level + WIS modifer. (The only reason I'm suggesting this instead of doing Intimate vs. intimidate is because if you are a high level character with low ranks in Intimidate the opposed check will be inaccurate. Can you imagine Rhissaerk being intimidated by a lower level char?
3rd Level Char to Rhissaerk: Hand over all your gold! (Rolls 1d20 result 10 + 10 ranks in Intimidate + 5 CHA modifier = 25)
Rhissaerk: ( Rolls 1d20 result 10 + 5 ranks in intimidate + -2 CHA modifier = 13) *eyes narrow* Sssstuipd dice! *Whacks 3rd level char over head with <insert current weapon here>, takes his gold and stomps off.
Concerning Bluff: The NWN engine doesn't incorporate the "sense motive" skill which is part of the opposed check for Bluff in PnP. As was recommended previously, I second the idea of using the Will Save since it incorporates your Wisdom modifier, not your CHA modifer as Bluff vs Bluff would.
Hope this helped
