So, back to my foolishness, in my original post, I claimed this as an advantage for Fan:
Scala has great support for Java, and it claims support for .NET, too. I've never used Scala nor Fan on .NET nor Mono, so I can't say which handles the platform better. So I'll just pretend they are equal on this subject (lacking time to gather evidence either way).
Side note, obviously GWT is more mature than either of these efforts, and for serious cross-platform support (though without Java or .NET so far), see haXe.