Following on from a64's excellent 'Photoshop Benchmark' thread,
I thought I'd post a quick summary (which is handy for all of those currently looking to purchase a new Mac, and interesting anyway for comparing machines).
(Taking a cross-section of Macs):
No.1 Fastest Machine

The Quad
19 seconds (wow!)
No. 2 (a tie) Dual-core 2.3 and Dual 2.7
39 seconds
No.3 Dual 2.5
41 seconds
No.4 Dual-core 2.0
47 seconds
No.5 iMac Intel 1.8 Dual-core
1:05 seconds
No.6 Dual 1.8 G4 Quicksilver
1:07 seconds
and so on ...
For comparison:
Mac Mini 1.25
2:36 seconds