I work at Microsoft full time on http://orchardproject.net, a CMS based on ASP.NET MVC, Autofac and Nhibernate. I've been doing that for almost 4 years and many would agree it's kind of a dream job as there are absolutely no constraints from MS to drive the project one way or the other. It's also worth mentioning that our team is much involved in OSS: SignalR, Entity Framework, ASP.NET MVC, Nuget, ... So it's a really bold bet on OSS in general.