Interestingly, Rdio 2.0 is built with C# and MonoTouch, sharing significant amounts of code with the Android client. https://twitter.com/migueldeicaza/status/273487637292130304
I'm still sitting it out but if WP starts getting any traction then that will probably be the last push I need to really dig in. There isn't a day that goes by when I'm coding in Obj-C or Java and don't wish for some C# feature.