I was watching the video of Miguel de Icaza's MIX session on Mono, and he mentioned a Java to C# converter called Sharpen. It is implemented as an Eclipse plugin. Interestingly, de Icaza indicated that the Mono team found Sharpen preferable to iKVM.NET (which converts Java bytecodes to .NET CLR bytecodes). I'm sure Sharpen is a tool I'll be looking at sometime, as C# and .NET are interesting, but I'm still far more familiar with Java.
By the way, de Icaza's session on Mono is very interesting in its own right, well worth watching.