Yeah, but add up the price difference, power consumption, and microstuttering... We got ourselves quite teh AMD fanboy here, don't we?
@op I have three HD6950s in CFX and they can hold out at 60FPS, but BF3 is quite an intensive game for CPU as well as GPU. A GTX580 would run ultra at maybe 50ish, as three 6950s are a fair amount more powerful than a 580, but mean more bottleneck and power consumption. If I overclock my CPU, I do get better FPS though.
You should look at 560Ti, or if you decide you want a bit more power, a 6950.