You have a Mac, you no longer need to use windows
*Waves hand across screen* These aren't the OS's you're looking for!
In all seriousness now, these guys are right. It would just be too slow. Even on newer PPC systems, windows and x86 Linux distros are pretty sluggish.
With programs like Virtual PC, and Q, running on a PPC (PowerPC -- referring to the Processer and motherboard that support it), the software must emulate a processor, the system that allows the processor to communicate with the virtual devices, as well as either provide a link from the software itself to devices (like your display card, network card, modem, etc). Sometimes these devices must also be emulated regardless. If you know anything about how processors are made and how they work, you already know that they are pretty complex devices. For software to emulate one, it's, well...slow.
This is why using VPC, Vmware, Q, etc on a windows box is a little bit faster. The hardware is already natively supported, and much less needs to be emulated. That's why the new virtualization, multicore CPUs, and higher bandwidth memory is such a big deal right now. We're getting to the point where we'll have the ability to simply run multiple OSs on a single system (at the same time), without having to emulate hardware, or create a seperate environment.
Probably more answer than you wanted, but maybe this sheds some light on *why* it won't work, or would at least crawl slowly (as if windows doesn't already do this
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