There's two issues I've seen with your posting Steve.
As a normal developer, you can't go from MS to Linux with ease. Unless you have spent serious time understanding the differences, strengths and weakness of the platforms, CLI and the tools, you will not instantly do quality, professional level work. It awhile to master the thousands of details that make a RDMS/language/utility/environment perform well and reliably.
Additionally, listing OSS experience on a resume for a Windows position or vice-versa is a instant disqualification or red flag when apply for a good percentage of jobs. (Bitter pill of personal experience...) There's too many entrenched managers that still see the software world as religious battleground. (You can thank Ballmer, Stallman and the fan boys for that...)
I do encourage admins and developers to spend some quality time learning different OSes, RDMS/NoSQL and programming languages. With virtual machines, free software and insane amounts of documentation, there's nothing lacking except some personal commitment of time. Many of the OSS languages and utilities are cross platform and can help solve problems faster, including that of a limited budget.