TIBCO Enterprise Message Service software not only provides a world-class JMS implementation, its .NET API supports and extends the JMS 1.1 specification. The TIBCO Enterprise Message Service .NET component fully incorporates .NET-style event handling and yet closely mimics the Java API. The implementation consists of fully managed .NET code.
I hear you gettin' all excited about this, but hold on. It is only for Tibco's enterprise messaging stuff. Supposedly, though, it works for .NET CF, too. Wow!
[ note added 3 Aug 2005 : IBM has announced an open beta for the IBM Message Service Client for .NET. ]