Didn't expect this, but MonoDevelop, the development IDE for Mono (open source implementation of C#, .NET, etc.) has been ported to the Nokia N900 phone (i.e., to Maemo). I mean, the full-on IDE. Now, I'm not sure how usable it is (certainly it's a case for using the stylus, all of the controls are so small), but I'm impressed about it because it means that Mono runs for Maemo, Android, iPhone, and Windows Phone. That makes it look like a good choice for writing cross-platform code for phone applications (albeit I know that they recommend you not do cross-platform GUIs, but the core processing code can be cross-platform).