I've been toying with the idea of creating a LLVM-based Clojure implementation, which would permit interesting constructs such as coroutines and continuations. Too busy though, and Java 7 will supposedly support them, if it's ever released.
MacRuby suggests that it's not a complete pipedream, but the experience of the Unladen Swallow team indicates that it will not be all unicorns & rainbows. You'd also have to find an appropriate GC (I know some people are looking at using Boehm to get MacRuby on iPhone, but you have to wonder if you'll run afoul of the Appstore Police by incorporating a GC in your app.)