It's probably a case of 'buy it before x date and it's free, after x date it's *price*', if there's any confusion. I think some iPod/Phone apps have done this before.
It's free to add it to your account until the 24th. After the 24th it goes back to it's regular price. Obviously once it's part of your account, it stays there permanently.
Passo: not that I'm aware of, although a bunch of guys have combined it with Team Fortress to make it multiplayer. Youtube "Team Portress" for a video.
There was a whole alternate reality game used to advertise Portal 2, it was amazing. It all concluded with that new ending and the announcement of the new game.