A patch was released tonight for Portal 2 that improves MSAA performance on all GPUs.
So I reran the timedemo on the top 2011 iMac versus top 2010 iMac versus 2010 6-core Mac Pro. GOOD NEWS: the 2011 iMac is faster than yesterday. BAD NEWS: It's no longer faster than the Mac Pro at 2560x1440. Here's the results:
At 2560x1440...
57 FPS = 2010 iMac Core i7 2.93GHz with Radeon 5750 (1G GDDR5)
83 FPS = 2011 iMac Core i7 3.4GHz with Radeon 6970M (2G GDDR5)
123 FPS = 2010 Mac Pro 6-core 3.33GHz with Radeon 5870 (1G GDDR5)
At 1920x1080...
85 FPS = 2010 iMac Core i7 2.93GHz with Radeon 5750
125 FPS = 2011 iMac Core i7 3.4GHz with Radeon 6970M
187 FPS = 2010 Mac Pro 6-core 3.33GHz with Radeon 5870
(Advanced Video settings used: 4x MSAA, Anisotropic 8x, Vertical Sync disabled, Multicore Rendering Enabled, Shader Detail Very High, Effect Detail High, Model/Texture Detail High)