If you're honestly just interested in learning another language for better job prospects, I would recommend staying a .Net guy and getting deep with Sharepoint

If you're honestly just interested in learning another language for better job prospects, I would recommend staying a .Net guy and getting deep with Sharepoint (I'm assuming you are already familiar with the new 3.5 framework. If you aren't, then start there). It's not glamorous work, but it is high-paying work. Otherwise, if you're really interested in startups, I would guess that Ruby is probably one of the more popular languages out there right now, with Rails being a very popular framework. This will depend somewhat on your location. Start checking Craigslist ads for jobs, that should give you a feel of what startups are looking for.