I no way work for StackExchange and I anticipate Jeff Atwood will chime in - however, I doubt this will happen. There is a similar discussion about this when the site first started. In the end this site works, it works rather well on the platform running it. We're no different or special compared to Webapps, Gaming, Cooking, or any of the other Area 51 StackExchange sites - so moving us to an Ubuntu server 'just because' would be like moving the Ask Differently site to an Apple server - just a laborious act that can only result in less than the current stellar stability and performance.
I'm not endorsing Microsoft as a solution - I'm endorsing StackExchange as a solution. I trust they will make sure the platform runs and runs as well as possible while staying online as long as possible. If that means running the servers on 1,300 Windows Symbian based cellphones then that's that. :)
StackExchange isn't OpenSource, isn't running on Linux, and likely never will. However, StackExchange is a community - and it listens to its community. The tools that run the site aren't nearly as important as making sure the community (and by proxy the site) grow to become the authoritarian Q&A site for Ubuntu related topics. That's where the SE team (and us) should and will be focusing time and effort.