[Update 23 Feb 2010: I've been informed that this was not a JIT, but instead a native-code generation backend for LLVM demonstrating LLVM's aggressive optimization potential. These numbers are not representative of JIT performance.]
Several people sent me a link to a very interesting blog post yesterday about using LLVM to provide native code generation for Haskell in GHC. I recommend it as an interesting read.
One thing I will point out is that the blog post doesn't really explain the whole situation. He shows plenty of examples where LLVM improved performance, but only mentions briefly that this isn't typical of larger programs and that most programs won't experience as much speed up, if any. So to anybody who reads this remember the caveat that the results aren't typical. JIT speeds some things up and slows some things down. It's not a magic bullet that makes everything better.