I play WoW on a 2.1 Ghz 256-X1600 MBP. I was getting 20-30 FPS in IF near the AH; so went to the WoW-mac forums, and did the things (turning off Full Screen Glow, playing at windowed mode with 1280x800 resolution, 1 x multisampling, half-terrain distance, everything else on high)...and improved it about 5-10 FPS on AH near SW/IF/Orgrimar.
MC/BWL/AQ40 raids usually stays around 20-25; fairly same as my 7800-PC. In non-major cities and world environment, I can easily get 40-80 FPS.
Check out the Official WoW-Mac boards for more tips!
EDIT: FPS should be fairly same between the 128/256 of the 1600's in the MBP. My friends MBP (1 gig, 128 meg Vram) with similar settings as mine has almost the same WoW FPS as I do. Patch 1.11 should have updates for better FPS on the intel and power Mac's.