And if you have played it before, there was a recent update that put some Easter Eggs in for Portal 2 promotion (like Morse Code, which reveals and old school dialup BBS address).
You don't need Steam's permission to play Portal. You can play in Offline mode (and really, Portal doesn't use online in any capacity anyway) with no problems. Almost all Steam games can be played offline, and that includes HL2. It's just a matter of clicking Steam-> Go Offlne.
The following link contains "spoilers" of a degree, but I am absolutely amazed by what's been hidden in the update for this game. There's Easter eggs, and then there are mind bending, stunning easter eggs.
If you don't want to play Portal, I'd advise you at least look at a play-through on Youtube. It has some of the most amusing dark humour of any game and of course, a totally unexpected and highly entertaining ending.
Plus it has Mike Patton in it, if only for about five seconds of incomprehensible noise.
I've had this since I got Orange Box for Xmas '08 (maybe earlier? I don't remember.) I loved the game, and now the wife is probably going to get it as well. I never did any of the hard modes, but I'll at least play it through again. I haven't played much Valve stuff since I beat HL2 Ep 1 a few months back.
Finally finished downloading it last night; and look forward to playing it sometime this weekend. Took me a few times to snag it from the steam-store, and click-through. First day it was available the servers appeared to be somewhat overwhelmed; I kept on getting try back later messages.