Check out these benchmarks. As expected, running (or rather, emulating) Photoshop with Rosetta is slow as shit. Like I told my friend Ming when he told me he was getting a MacBook Pro, if you are getting one to do Photoshop primarily, you should wait until Adobe releases a native version of PS CS2 for the Intel-based Macs, only until then we would be able to see benchmarks and decide if it makes more sense to get a Mac or a PC.
If you want a MacBook Pro because of OS X, the iApps, or anything other than for Adobe apps, then go ahead. 🙂