Further to some previous answers; some other Mono details:
Currently the compiler targeting .NET 2.0 and 3.5 is NOT the default compiler. To me this suggests an immature platform. If you're starting out a new project, being able to use all that the .NET framework has to offer reliably is a huge plus. IMHO, Mono doesn't bring that to the table if you need to push the .NET framework to it's limit/edge cases.
Mono is sponsored by Novell, which does speak of its potential longevity as a platform.
