Recently there was a study done on "social gaming", where you earn prizes for working out.

After 10 weeks, those who played the games exercised more often and lost more weight.

Here are 3 examples of "social gaming" apps, all free on iPhone or Android.

Nexercise -- Every two minutes you spend exercising equals 10 points.  You redeem points for things like Amazon gift cards.
GymPact -- Charges you money if you DON'T work out.  But If you DO, you win cash.  You decide how much to penalize yourself, and "check in" every time you work out.  If you DON'T, you lose that day's cash.  You get a few bucks when you DO meat your goals.
HealthRally -- This one lets your friends/family pledge money for rewards like iPads and running shoes, but you only get them if you reach your goal. If you DON'T, then no one's credit cards get charged.  (This can also be used to help friend with stuff like quitting smoking!)

Happy earning!...er, um...getting healthy!