The problem with .NET is that Mono isn't a port, it's a community reimplementation, and AFAIK, the only part that Microsoft even came close to sanctioning was Moonlight (Silverlight for Linux), and that's because they were desperate (not that it got them anywhere :P). Because of this, while C# itself will work on Linux (and it is a nice language to work with - I used to use it, and I liked it), .NET apps aren't always portable due to Microsoft-specific libraries and such (the Ink libraries used in Paint.NET come to mind). So yes, you can use it in Linux (and thus theoretically on Maemo), but no, it's probably not the best choice, and no, it won't always work smoothly. I can understand why you'd want to use C#, but if you're serious about Linux (or cross-platform in general) programming, you'd be better off picking up something more universal. I don't know what would be the best for your application, but for the N900, C# wouldn't be it.