Interesting review and even more interesting benchmarks. At first glance it looks like the iMacs outperform the Mac Pros in most tests. Digging a bit deeper though, both Mac Pros tested are in base config with no extra ram, and the stock video card.
http://www.macworld.com/article/143970/2009/11/core15_imac.html
http://www.macworld.com/article/143970/2009/11/core15_imac.html