There has been some work done by Pierre d\’Herbemont [1] lately on QEMU to run on OS X [2]. QEMU [3] is a fast processor emulator using dynamic translation to achieve good emulation speed.
[1] http://mail.gnu.org/archive/html/qemu-devel/2004-02/msg00104.html
[2] http://cvs.opendarwin.org/index.cgi/projects/darwine/related/qemu/
[3] http://fabrice.bellard.free.fr/qemu/
