Bitcoin C# is a port of the native Bitcoin P2P protocol into a C# library.
Bitcoin C# makes it easy for C# application developers to add bitcoin features to their applications.
The goal is to make Bitcoin C# the best C# implementation of the bitcoin protocol.
=> NEW 6/19-2011, after putting the project on codeplace I found out that another port had been done. It is well done so I recommend you go there http://code.google.com/p/bitcoinsharp/.