A hacked, named quarnster, is working a xbmc version for the Boxee Box. At this moment only the GUI is working by EGL/GLESv2 rendering, but hardware accelerated video and audio is a work in progress. The only requirement for now is root access. XBMC will be installed side by side next to the Boxee binaries, so limited risk is involved at this stage.
This project sounds very promising and could be a real alternative since Boxee won’t bring out any more updates for the Boxee Box. The project is open source and hopsted on github. Installation instructions can be found here: http://github.com/quarnster/boxeebox-xbmc