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As of July 2006, the C# code generator and runtime are almost ready for ANTLR v3.0b3. However, development proceeds sporadically due to other commitments.
The C# code generation templates and the C# runtime library itself are essentially feature complete with respect to the default reference Java version. There are currently zero automated tests written since and it isn't yet clear what is the best way to write how such tests can/should be written (we suspect this applies to all the other targets too). In any case, some basic sanity check are done with samples grammars in the examples-v3 archive.
Architecture
The C# code generation and runtime are currently based on the Java version and would remain so until well after the final release. ANTLR 3 development (including AntlrWorks debugging support and some of the internal interfaces) is still so fluid that investing significant effort in developing radically different code output would be a little premature at this stage. That would be left as an exercise to the reader 
Target Platforms
Microsft Microsoft .NET v1.1 and v2.0
Mono v1.1
Supported build tools
Microsoft Visual Studio 2003 and 2005
Nant v0.85
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Should be easily faster than V2 C# code generation. I doubt it would be able to outrun Jim Idle's C target output or Ric's C++ target but, it should be competitive with the other targets.