I tried this late last week, on a spare P4 we has around at work, and it works. And works quite well *but* rosetta the emulation layer simply cant handle any apps that specifically require a G4 or G5. This throws any intensive video editting tools (such as final cut pro) out of the window but doesn't mean you'll still be able to use apps like photoshop and firefox.
If you want to try you'll also have to endeavour the pain that is installing OSX through PearPC, not a pretty sight. Especially if you are using Tiger.
8-)
