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There is Mono for Android, the .NET framework ported for Android. And there is MonoDroid, a development stack for using C# and the core .NET APIs to develop Android-based applications. MonoDroid Preview 1 has been released a couple of days ago.
.NET and Mono are great environments, with many tools and and excellent skills base of people who know how to use them.
I think Mono has the opportunity to be the mobile cross-platform development environment of choice, seeing as they are the only alternative to Objective-C on the iPhone and should be portable to Android, and .NET is already on Windows Mobile.
I'm really hoping to see a solid implementation of Mono on Android, with wrappers for the Android API as with Monotouch, and would be prepared to pay for it since I'm not in a position to do it myself.
You're more likely to see an Android implementation of Silverlight. Microsoft rep has confirmed that it's possible, vs. the iPhone where the rep said it was problematic.
But a version of the .Net framework is possible. Just need someone to care about it that much :)
But really, moving from C# to Java isn't that big of a deal and considering the drastic differences between the two platforms (PC vs. G1) it seems unlikely that you'd be able to get by with one codebase for any app that you wanted to run on both.