Me and some friends are considering writing a new FOSS project, and the debate is divided between using .NET or something else.
So I am wondering if there is an advertis
Your best overall bet is to go with .NET. While we are focusing on getting the application suite developed, we are keeping technologies as cross-platform as possible (I would use the term generic but that would be confusing).
Get your overall design/architecture in place first. Then study the technologies you are considering. For instance, if your application is going to target a Mono platform, there are MS technologies that you shouldn't even consider (MEF comes to mind).
Don't let a Linux/Windows/Mono discussion keep you from getting anything accomplished. ;)
Edit based on Am's comment: I didn't realize you were actually concerned whether the decision to use C# was the best decision. In light of that, I'll add a bit more.
Mono is written in C#. The popularity is overwhelming. Add C#'s flexibility and an extension (more or less) of C++, I really don't think you can go wrong. There are still considerations as I mentioned above that you need to keep aware of.