I love this technique California Pizza Kitchen is using. This article asks if we should be doing more of it in interaction design. I would say yes if you are having a hard time getting repeat visitors or users.
“Here is a rather interesting promotion from California Pizza Kitchen. At the end of my dinner (along with the bill) I was given the “The ’Don’t Open It’ CPK Thank You Card.”
It’s a coupon, with an interesting twist: You bring this with card you next time you come back to CPK. You’ve already won something, from a free appetizer up to $50 dollars (or more). But you don’t know what you’ve won until your next visit (see where this is going?). The instructions are pretty clear: Whatever you do, do NOT open this or whatever you’ve won is null and void! A manager has to open this for you when you return. You are guaranteed to get something worthwhile—and this is a critical part or arousing curiosity. Coupons are too explicit—”here is your 20% off.” Scratch offs and lottery tickets are most likely to reveal nothing. Here, the fine print teases you with a list of the possible prizes. Now I’m curious: which prize have I won? This is a mystery that needs closure.”