"After all, what's the difference between legislation mandating open source, and legislation outlawing P2P file sharing? In one case, we say that it's good, and the other that it's bad."
A lot, actually. At least in theory, governments are there to make laws that make life better for the people they represent. This statement sounds like you'd really prefer that governments do nothing - an interesting argument, but not one which I think has much to do with open source per se.
We've got an opportunity now because the proprietary vendors have discredited themselves on a number of fronts - Microsoft in the antitrust trial, Oracle in California - so I don't see much reason to be particularly paranoid about government action on this front now. In fact, I'd argue that it's probably the best moment we're likely to see.