I had a terrible thought the other day. You know sometimes you buy that game in the store that you know should be good but never quite get around to playing it, and it just sits there unopened gathering dust on a shelf? I was thinking, would it get to the point where the Steam key inside would expire and I wouldn't be able to activate the game when I actually wanting to play it? I tried googling this and looking it up on Steam but I can't seem to find the answer I'm looking for.
To the best of my knowledge, the answer is no. Should the Steam client ever be taken down for whatever reason (company bankruptcy or some other bad situation like that), I believe they once went on record as saying that they'll provide a way for people to download/play their purchased games without the need of running them through Steam.
No, they would not. It is a recommended to keep the a long time though. Sometimes hackers would release a lot of Steam keys for a game, and if you activate the key, the game becomes invalid. It happened with Dirt 2 if I recall correctly.