http://www.teamfortress.com/mvm/faq/
So this thing is ready to go, and here are the highlights:
It's free to play these missions, however, to be able to earn items and rewards from these coop missions, you must buy an admission ticket for $1 US dollars (or equivalent).
As a free player, I am glad instead of sad. Why? Because I barely have enough backpack space most of the time and there are these vouchers that force you to get like 6 items at once! No thank you.
So basically all f2p's can do is play the coop mode, but with no rewards and badges. The rest of you still need to pay $1.00 to play the 'official' mode that has the rewards and stuff.