For the iOS (iPhone and iPad) I recommend
MonoTouch[
^] and the
main site[
^].
For Nokia, that is a tricky question, but for Symbian the best way is a
MIDlet (Java)[
^] it does not only target Nokia and Symbian but others mobile os that supports MEDlet.
But Nokia is moving to Windows Phone 7 from Microsoft.
For HTC (Android) you can play with
MonoDroid[
^].