Does an old AppleCare package expire?

I bought AppleCare for my iPhone 3G back in the day, but never opened it so it's still factory/plastic-sealed. I have an iPhone 4 now but my window period for extending my warranty coverage came and went.

Do unregistered AppleCare plans expire at any point? If they don't, I'd like to sell this, make some needed cash and help somone else out.