I'm really excited about this project. Between xargo, cross, and dinghy we'll soon have very good build/test support for a large number of cross-compile cases.
It would be great if we could do Rust's own CI with cross. Solve that problem once for everyone.
It seems like cross could probably do Android, if you can work out the license issues.
It could probably also do Windows, GNU with MinGW, MSVC with Wine. Maybe OS X with a custom built clang.
It'll probably all more-or-less work on Windows too with Docker on Windows.