Your post has prompted my curiosity so i tried to share my winxp sp3 guests run by vmware fusion to my 360.
here is my setup: i have a macbook running fusion with winxp sp3 (bridged mode)
i also have a imac running winxp sp3 bridge mode with fusion
all connected to linksys router (xbox to router as well)
firewalls are enabled on host, guest and router.
i have connect360 on both imac and macbook osx and they are able to stream from mac without issues
on the guest side, go into wmp11, click on tools, options, library, sharing, click on enable sharing, rename share name to something unique (ie. not default OWNER). make sure to maximize media type shared, ie. don't limit to a certain star # (that was my problem, once i clicked on all ratings, it solved the issue).
i know it sounds silly but that last point made all the difference. give it a try and let us know
Mike