The new
Aperture Mac app might make a Mac worth looking at for the pro photographer, depending upon your needs. The only problem with buying a Mac right now is that the G4-based systems are seriously obsolete and will soon be replaced with Intel-based versions that will be much more powerful and possbily even thinner (EnGadet's latest Podcast mentions rumors that the Intel-based PowerBooks will likely be a lot slimmer than the current models).
Hardware-wise it is bit hard to justify the G5s right now, since it is really easy to build a killer Opteron 1xx-based dual core system that has the same specs as the entry level dual core PowerMac for something like half the cost. If you shoot a little higher for around the $1999 price of the 2.0GHz Dual Core PowerMac you can easily turn that Opteron into a pretty formidable and rock-solid stable machine (2GB of Corsair RAM, P180 case w/ PC P&C SLI 510W PSU, GeForce 7800GT, Tyan server-grade mobo) that would humiliate the base configuration of the G5 PowerMac when it comes to Photoshop work.
So it kinda depends on what is more important. Apple has a better selection of professional software, but in the Windows world you can buy a lot more machine for your dollar (especially since the 100 series dual cores are so cheap now).