How long is this going to last? I have no idea. For that reason I recommend that you run to Steam as fast as your virtual legs can carry you. Download Limbo and enjoy one of the most interesting video games of the last decade.
Why is it free? If I had to hazard a guess I'd say it's something to do with Playdead's next title, Inside, being very close to release. Inside is scheduled for a July 7 release and, by all early accounts, is a very good game indeed.
Therefore it is probably a good idea to prep for that game's release by playing Limbo. For free.
I repeat: for free.