While resigning on principle is almost never a good idea, I actually did that at the last company I worked for when they've decided to move to .NET exclusively, even tho it didn't affect me directly - the existing small 'division' of .NET guys were such asses and our ways of thinking were so opposite that I figured it would only get worse when the new .NET blood (read it Microsoft mindset) starts rushing into the company... Not to mention that I don't like the .NET premise anyway.

Turns out it was the best decision of my life - a month later I got a new job, got almost double the salary, leading the R&D department and having an equal vote as the managers when it comes to strategic decisions. Epilogue: the company I've left is now reduced to a 10-people team that my company from time to time outsources for some ridiculously low cash; I have twice as much free time and I'm actually doing something I love; Sometimes it really does pay off to follow your principles.

Best decision of my life, period!