i've been a long-time user of XBMC (and sometimes Boxee, but it's full of bugs..), and i wondered: it's open source, it's a stable software, and yet it's not in USC. why?
XBMC is now included in the Ubuntu Software Centre for Ubuntu 12.04.
See http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=469397 for an in-depth explanation.
The licensing issues have been fixed, but nobody cared to bring it into debian since then. Ubuntu is mostly build on debian packages.