I’ve never seen Sony do this before, but if you download Fat Princess: Piece of Cake (a new game in the Fat Princess series that was just released) and play it and complete level 15, Sony will send you a voucher code for a free copy of Fat Princess on the PS3. That’s a pretty neat bonus, not to mention that Fat Princess: Piece of Cake is also free to play as well.
Bonus PS3™ Game!
Escort your beloved princess to Level 15 and earn access to a download of Fat Princess for your PS3™ system at no additional cost. You must complete Level 15 in Fat Princess: Piece of Cake and have an active Facebook account to receive the voucher code. A valid Sony Entertainment Network account is required to redeem the voucher code and download the game to your PS3™ system.
UPDATE: Also seems like if you play the iOS version, you can also get a voucher code for reaching level 15.
[Source: PlayStation Store]