Virtualization is a great way to improve reliability, take advantage of hardware and scale. For example Mozilla’s build team uses it to manage all the build instances that used to be on individual machines. These servers essentially compile code all day long. One problem with virtualization and cross platform building is that Mac OS X doesn’t run in any virtualization environment (because of Apple’s interest in selling hardware). This means while you can run Windows and Linux on the same boxes, you still need to have and maintain separate Xserve’s for the purpose of compiling for Mac OS X. Looks like Mac OS X Server 10.5 (and only server edition) now has a license that permits running virtual. While great, this makes it pretty expensive to do things like a build farm. You can’t just buy a Mac OS X client, even though that’s all you really need. You need to buy server.
Currently, there’s nothing other than PearPC that can run it (and PearPC is worthlessly slow). Hopefully VMWare will update at some point to support it. At that point, things can get interesting.