Our game is currently only Direct X 9. We've done everything possible to make it look great for DX9.
We use a Deferred Lighting model that most other game don't use. Ill try and get some more info up next week since the technical details since this will probably be interesting to many of you. AA does not work very well with our lighting model unless the game is DX10.
For the forums it's clear that there is huge demand for AA support, so we will be looking into it.
This is just one of the replies on this thread from Rockstar Toronto. They are actively involved at solving our issues on that forum.