EliteBastards talked with ATI about the possibility of using anti-aliasing in Ghost Recon: Advanced Warfighter:
The DX9 spec doesn't allow multi-sample AA when using MRTs and our hardware requires that all of the destination surfaces either have AA or not. This means that in order to get AA in G.R.A.W. we'd need to have lots of AA surfaces and perform a ton of AA resolves. The end result would be slow and require much more texture memory. It's not 100% impossible, and I'm not giving up on the possibility, but there is no playable solution right now.
So, there you have it - There is still a chance of being able to use anti-aliasing in Ghost Recon: Advanced Warfighter on ATI hardware, but seemingly only a very slim one. Our assumption was that the use of deferred shading for the game's lighting technique was the prime suspect, but it seems from ATI's own analysis is that it's the use of multiple render targets that cause the issue here.