Microsoft's Secure Boot feature will be mandatory on ARM-based Windows 8 tablets, according to a discovery in Windows hardware certification documents just found this weekend. While it will be optional on x86, disabling Secure Boot "must not be possible" on ARM. As described, it would prevent any unsigned operating system from running on the resulting hardware, including Linux and variants on it, like Android.
Linux could technically run on these devices if it was signed, but it would also be violating the GPL (GNU Public License), the term needed to be considered an open-source platform. GPL code usually needs to be redistributed, which isn't practical when even the Linux kernel would need to be signed. Every unique variant on Linux would need a code signing from every hardware manufacturer.