Wow. I'm no hardware hacker, and my knowledge of the internal mechanics of OSes is pretty thin, but how is that done?

While skirting the issue of legality.. suffice to say there are developer versions of OSX in x86 format. If you happen to have hardware that is similar to the x86 machines Apple 'leased' to developers then the install works just like a normal OSX install on a PowerPC based system.

Dang; it seems then that it's unlikely it will be possible to run a triple boot of MacOS X/Linux/WinXP on my Athlon then. I guess I'll keep saving up for an Apple laptop so I can have a the trifecta of development platforms.

Well it's possible depending on which chip you have. For a working machine supposedly all you need is an x86 processor that supports SSE3. This isn't a rule though, as the P4 I used was a Northwood core so it only implements SSE2. Rosetta (the emulation technology for running PowerPC apps on Intel hardware) is the only thing that *requires* SSE3.
There are some people out there running it on Athlon64 chips and other products from that family. It also seems to work well on nVidia based mainboards. This is all dubious at best though since Apple doesn't support or condone using the OS this way. I'm sure they don't mind the publicity though.