> Well Google already did that with Java and GWT, but it never caught on.

ClojureScript uses the Closure(sic) compiler from GWT to compile Clojure/Java to JavaScript (not that I defend this technological train wreck).

> I suppose it is a fair question why they didn't just use C#.

This would have been superb, but I think Microsoft wanted to show that they "get" web developers; just mentioning C# causes the JS pundit get jittery because someone wants to limit the amount of write-only code he is able to spit out.