Portal was free, at least for a few weeks... although it was for the whole Mac thing.
Either way, I don't think those other games would go F2P... even though Portal 2 has a store of its own, the profit from there alone isn't that great. I imagine.
When the free weekend is on, the game will be at half price.
I'm using a guest pass and today when I quit the game the "the fun doesn't have to end" popped up.
http://img682.imageshack.us/img682/7613/49717734.jpg