Dutch software developer Jeroen Frijters is developing a Java virtual machine for .NET and documenting its progress on his Radio Userland weblog. His reasons for going to these lengths, rather than using J#, are interesting:
"The J# object model is fundamentally broken. In order for it to function, it requires a huge security hole. I reported this to Microsoft and gave up on the tool."
This 1999 bio of Frijters makes it sound like he's got serious Windows and Java programming chops.