As much as I dislike .NET in general (there are some excellent parts) and doing large enterprisey development, I think C# is an excellent language that is only getting better. C# started out as a clone of Java, but it seems to be what Java could have been if it had evolved more rapidly.

If you're concerned about concurrency support, there are new features in .NET 4.0 that I find pretty impressive and nice to use.

That said, I don't know if the story for developing C# applications on Mono is as solid as Visual Studio.