It's the fastest Mac ever, except that it's not.
About 1/2 of the Macworld tests are hard drive intensive tests (duplicating files, importing files, zipping files, etc). Of course the SSD based iMac will outperform the Mac Pro on these tests.
Comparing the CPU/memory performance instead and you'll find that the iMac gets a geekbench score of 11648, while the Mac Pro gets an astounding score of 24262.
What's even more deceptive is that the SSD in the iMac is not even a high performer (220MB/s read 180MB/s write) -- it's simply better than a stock hard drive. For the same price ($500 BTO upgrade vs $550 total cost) you could get a 3rd party SSD from OWC that gets 559MB/s read and 527MB/s write.
Heck, any Mac that supports SATA III and had an OWC Mercury Extreme 6g installed would be declared the "fastest Mac ever" according to these messed up Macworld tests.
edit: Macworld compared the 2011 iMac against the 6-core Mac Pro, making the headline "Fastest Mac Ever" even more ridiculous.