A question for the collective wisdom of LJ

How does one authorise credit card transactions in the US - is it signature or PIN? If it's PIN, and you don't know the relevant number, does that render the card useless, or is there a way around it?

And while I'm posting, happy birthday to the five million people on my flist who have birthdays today! (That's undyingking, kesstrel and marnameow, I think. And happy birthday killalla for yesterday. Edit: and condign and cuthbertcross! Wow, that's a big clump o'birthdays ;-)