Obviously you can't really run Mac or Linux apps on Windows, but can you compile binaries for those platforms using MSVC++ for example (plugging in additional compilers and tools obviously)? For a serious build system, you don't want one build server per platform so having an automated build server which compiles for all target platforms seems quite a reasonable aim.
Crosstool-NG seems like your best option for Linux apps; they show that as one of the standard configurations. I do not know about Mac OS X; this question suggests that it will probably be difficult.