On the http://forums.steampowered.com/forums/showthread.php?t=1265332, at the bottom, a (note: volunteer) moderator stated that it will remain on your account for free after the 24th.
But if you ask me, it's all very confusing. But portal doesn't show up in your library until you "buy" it so it doesn't work like the free weekends and when you get portal it gets registered to your account (I just installed it on my brother's pc) so I guess it might actually remain free if you get it now.
It's probably to give mac users who didn't have a steam account yet a reason to keep steam installed with the hope that they'll buy more games for their account in the future.
And given that they'll be release Portal 2 http://developer.valvesoftware.com/wiki/Valve_Time, its hardly going to hurt their profit margins. Overall, I'd say its was a great decision, business-wise (not that I'm in any way qualified to make a proper assessment of such things

).