Graham, My last two employers, both new companies, embrace open-source solutions (but themselves produce proprietary solutions). Doing web programming, I was fortunate enough not to have to touch a Windows machine to do any coding, although good Windows programmers don't have trouble finding work out here (I live in California). I think increasingly, that open-source is being looked at as a viable or preferred development solution, and certainly the comprehensive power of Unix and Linux is compelling for many developers that may be involved in decision processes at companies. My current company prefers open-source developers, but that may be more of a preference, and one of the luxuries of being a small company. Open-source-preferring companies are rare, but then so are open-source developers (or so I feel).
The rest of what I have to say about your question is mostly conversational, but I'd have to say that the situation out here is not as bad as you might think. If the company is good, and you're a good fit, they wouldn't think twice about sponsoring you. I think losangeles.craigslist.org is a good place to browse for jobs. Best Regards, David Beal