As the Mac OS X fanatic it is hard indeed, the only hope is the Rotor (by Microsoft) and Mono by bunch of Aussie yahoos.
I wrote code in C# when the first .NET framework came out in Beta version (before Visual Studio .NET) and then J# and C# in ADEXS since their platform is based on .NET, but for last few months I enjoyed the bliss of Java on my Mac.
Well, I have to admit publicly that C# is awesome language and Visual Studio .NET rocks compared to anything Java (I still use NetBeans!), we will see what come out of that.
I am writing on Mac OS X emulation of Windows 2000 Pro (Visual PC 6).
(Update: Since this post Java frameworks and new features surpassed C#. The .NET Mono/Rotor never made it to Mac world. Java Groovy/Grails blew anything else out of the water in features.)