Looks like JetBrains (IntelliJ) are considering doing a C# IDE. Spotted this on the early access newsgroup:
Noticed intellij are advertising for a C# developer on the .com website.
Does this mean a C# version of IDEA is coming?
It's coming if we find those great C# developers :) Yes, we are planning it.
Eugene Belyaev, CTO
This is great news. IntelliJ IDEA is by far the best IDE I've ever worked with; it's jam-packed with many useful automated refactorings and tons of other features that make a developer's life much easier. If they can bring that to C#, I may just have to retire my copy of Visual C#.NET.