Mine until unprofitable then resell for an ebay premium?
I'll hold onto mine in case there's a difficulty crash, but it's always an option that you can work into your roi.
Has a difficulty drop ever happened? I'd figure unless they decide to make more bitcoins available that it would be the only way to make a difficulty change.
Yes, a difficulty drop HAS happened before. Quite a big one too. Look at the chart history of difficulty.