Pretty sure we're still waiting on benchmark results for the 2011 i7 iMac, but the 2010 i7 iMac already had a faster processor than the 2010 base model Mac Pro.
Correct me if I'm wrong, but in theory, in terms of raw processor power, we've got the 2.8Ghz base model Mac Pro with 4 cores. That's ~11.2Ghz
iMac is 3.4Ghz with 4 cores, so 13.6 x + ~15% for next gen processor, so about 40% faster than the base Mac Pro from 2010?
That would be assuming that graphics cards, memory, and HDD factors are irrelevant, of course.