Macworld just put up some new benchmarks for the new 17" and 20" c2d iMacs. 10% improvement over previous models (doesn't include the new 24" imac with faster processor and graphics though). And then there's this:
More significant, the 2.16GHz system narrowed the performance gap between iMac and Mac Pro product lines. With twice the number of processor cores, all running faster than the iMac, the Mac Pro had a definite advantage in this match up. But because not all applications and tasks take full advantage of the Mac multiprocessing capabilities, most results showed the Mac Pro between 20 and 30 percent faster than the 2.16GHz iMac. I expect that test results of the new 24-inch modelwith its faster graphics and the optional 2.33GHz processor upgradecould close this performance gap even further.