Using your settings at 2560x1600 I am seeing some hitching on dual 5970s - this applies to every level of MSAA for A-tested objects. Not a huge game breaker, its playable, but not ideal.
Reducing the overall MSAA to 2x reduces the hitching; turning off MSAA eliminates it altogether. If I were playing this game, I would probably turn off MSAA and run everything else at their highest settings. Minimum FPS with MSAA off was also much improved (40-60FPS) vs. 2x or 4x MSAA (in the low teens.)
I also tested Catalyst AI standard vs. advanced, and normal AA vs. Adaptive AA; no real difference in the 'hitching' experienced.
I ran the test using 16xAF and texture filtering set to 'highest quality.'
Based on the testing, I think this is a frame buffer issue, with the hitching resulting from VRAM-to-RAM memory swaps or something similar.