Slight difference in performance, but for those who demand every extra fps they can nab the 2.4GHz MacBook Pro w/ 256MB of VRAM will still be worth the extra cash. 🙂
Either way you go, you still get one heck of a fast laptop. 🙂
It depends what you plan on using your machine for.
For example, if you run multiple displays, it might be worth the extra. If you run any kind of batch jobs (say, 3D rendering) that take a long time, it might be worth it. If you want to play lots of current Windows games (via BootCamp) and want to keep the settings as high as possible, it's worth it.
None of the games tested are terribly stressful on a system when compared to recent Windows titles (e.g. Oblivion, S.T.A.L.K.E.R., Supreme Commander, Company of Heroes...) -- if some games like that are tested I think we'd see a greater difference.
Having said that, I doubt the 8600M won't be running those games well with full eye-candy anyway.