From Apple's XCode 2.2 User Guide:

Computers that compilation tasks are distributed to must be running the same version of the compiler specified by the target and the same operating system as the build host. These computers must also have the same architecture as the build host. For example, an Intel-based Macintosh can distribute builds only among other Intel-based Macintosh computers. A PowerPC-based Macintosh can distribute builds only among other PowerPC-based Macintosh computers.

Well there goes that idea...