Much has been made of the fight in Wisconsin about collective bargaining, and the teacher's union. I don't agree with Wisconsin's laws that essentially require 100% membership, since they're not a right to work state.
I completely support their union's right to join together and collectively bargain. Let them get together and decide as a group whether they want to accept a pay offer. I personally consider unions the pinnacle of libertarian action; it's the people working together.
However, there is always the "or else". Unions say "pay us X or we all quit". Employers say "I'll pay you Y or you're locked out". The whole system works for most employers because the jobs are just skilled enough to be very expensive to replace, while not so skilled that you can never replace them.
My advise to Walker: make the Y offer, then fire them. There's a lot of replacement teachers floating around out there.