A few tips I've found since GT2 (the AI hasn't changed much)
- Go inside, not outside - the AI will almost always drift up inside of you if you try to pass on the outside of a corner.  If you are most of the way around this means hitting your rear quarter which will cause a spin.
- Avoid sustained contact - the AI doesn't know what to do if you are physically touching them.  If you happen to bump another car from behind, sometimes they will just coast for a while.  You need to immediately back off and get away from that car so it can continue and not impede your progress even further.
- The AI is on rails - you canNOT spin the AI or knock them substantial distances.  This is why inside is better than outside.  You can use this to your advantage (cheat), but be very careful.
- Small bumps can be effective - you must be parallel to the AI car when "bumping", but sometimes you can nudge them just enough to take the inside or make them alter their course.
- The AI will back down - they can recognize other objects (you) and move, but they can't do this quickly.  You need to gradually assert position, and sometimes the AI will move.
- Don't take the straights for granted - just like you can get spun in the corner, the AI will sometimes make a move (for no real reason) into your rear quarter and send you sideways at Daytona going 330kmh.
- Give them space - they are unpredictable.  When passing, don't try to trade paint.  Try to give as much space as possible and make sure you've successfully completed the pass 100% before moving back onto the racing line.  A lot of AI accidents happen when you dive bomb another car, get most of the way in front, but can't carry speed through the corner and end up sliding up into the other car - they clip your bumper and you spin off the track.
- Intelligence is artificial - as unpredictable as they are, they are entirely predictable...not sure if that makes sense.  Basically, they are programmed and will act in a calculated manner.  Even if you don't think their behavior is realistic, notice patterns and use them to your advantage.