>wait, how can you run it? i'm just speaking hypothetically here.
The story goes like this: before Intel Macs was released and OS X Intel was in the beta testing stages, Apple rented out special "developer boxes" (with the promise that the developers would return them after the Intel Macs were released), which were machines very close to regular PCs, and came with OS X preinstalled. The purpose of these boxes was to allow companies to test and get their software Intel-ready before the Intel Macs were actually released. One of the main differences between the developer boxes and current Intel Macs now is that those ones had BIOS instead of EFI.
So, what happened was some developers of course pirated the pre-release version of OS X (which had many security holes), and released it on Bittorrent. By using this buggy pre-release version of OS X, and building a custom PC that's similar to the developer boxes Apple made, it's possible to run OS X on a PC. But overall it's quite unattractive, mainly because any updates applied to OS X would render it unbootable.
>no, it wasn't that, it was a special hacked version for AMD.
Ah yes, that's using the prerelease version of OS X and a kernel hack, which got it running on VMWare (as well as being possible to run natively under the AMD processor with a different hack from the osx86 project).
An update: I've just recently discovered that PearPC has a new site, which is indeed active.