Well, this is irritating. I'm having false negatives from Steam Guard.
I've been flitting between my desktop and laptop while the Portal ARG has been going on; desktop to play games, laptop to leave them running for long periods of time. Most recently, though, I've not been able to access my desktop; I get the Steam Guard warning, it sends me the mail, and then it doesn't accept the access code from the mail.
It's not a case of typing it wrong, it's failing even if I copy and paste directly. It's just not letting me in. And - presumably because it knows it's already sent one - it's also not sending me an updated one to try afresh.
I've poked Valve about it, at least, but it's certainly mildly irritating; hopefully the problem can be ironed out before I take my long break off work for Easter, 'cause I was looking forward to cracking open Portal 2 then.