This is probably in a move to get more people on to steam than ever before. On one can deny that a free game is not enticing, especially a game that has such positive buzz around it.
The game itself is brilliant, only one game has ever made me laugh more at it's jokes and writing and that's Portal 2. Not only is it funny, it's seriously fun to play, every time you play feels rewarding in some way. It's defiantly worth a try for anyone who has yet to play it.
while this makes my two gifatable copies worthless, I really hope this get more people to play it. Portal is a great game that deserves all the praise that it gets. If you are reading this and have yet to play Portal, go on over to steam and download yourself a copy. It doesn't even require a hefty PC, it's a game that my 8-9 year old pc was able to play pretty well, so that's no excuse.