I'm in the same boat. It's a shame too, in my mind at least, because I really do enjoy working with the better parts of the platform (MVC, WCF, C# 4, VS 2010, etc).
I'm currently working in a .Net shop and the work is not terrible by any means (it's a fairly large scale web application used by school districts), but I'm starting to fall out of love with the MS lock-in and general attitude of the folks I've worked with so far. I'm pretty sure 95%+ of my current and past co-workers have no idea what hacker news is (or even /r/programming for that matter). The general reaction when telling someone about my side projects learning Scala/node/rails/etc on my (gasp) Mac at home is along the lines of "huh?".
That was years ago though... and since then I've happily moved on to being self-employed and using OSS.
Eventually I got tired of it and went back to primarily Win32 development for customer-installed apps and Linux for everything else. I feel like I can focus on quality rather than quantity much better now.
It's that which is mystifying, not the zealotry.
If you had to choose one of two faults: Nothing But Microsoft OR Anything But Microsoft -- you're going to be better off as an ABMer since (theoretically) your ecosystem of options is MUCH larger. Rejecting Microsoft still leaves you with several other marketable options. Rejecting everything that isn't Microsoft leaves you completely at the mercy of a single platform that you have no control over (and if you did have control over it, it still wouldn't be ideal)
So on one hand you're dealing with a fanatic that only sees black and white, and the other you're dealing with apathy. Not exactly good options.
(As noted in a sibling comment, it's also very common to see people turn their brains off rather than entertain .NET as a sometimes-appropriate tool.)
The reception to ASP.Net MVC, though, confirmed my original point about the .Net community. I heard lots of talk treating the MVC pattern as revolutionary, when, in fact it was something that was rather common outside of the .Net community.
But it takes the Microsoft stamp to get a lot of these people excited.
Yes, there were other "Rails for .NET" frameworks out there. But, you are correct: the vast majority of .NET developers are waiting for the official Microsoft seal of approval before adopting anything. It's part of the culture that comes with choosing Microsoft for your stack: you want support, or at least somebody to sue when it goes wrong.
That's a valid concern, in my opinion. I don't know why, but even talented Microsoft developers I've spoken to have been woefully unaware of developments outside of the .Net/ASP sphere. It's almost as if innovations don't exist until they've been incorporated into the CLR. I've seen similar attitudes in Java developers, but it appears to be far worse in the .Net community.
That was, in fact, one of the reasons I chose to keep practicing other programming languages while I was employed by a .Net shop. I didn't want to end up "stuck" knowing only .Net like so many of my co-workers.