Very cool, but the experience was ruined when I was testing a game I made and was interrupted by a lightbox saying "Seems like you have done a nice game! If you want to continue making awesome games and saving them, you should register! It only takes 10 seconds." I get the reasoning for this, but I'd suggest a lighter touch for prompting users to sign up -- namely one that doesn't interrupt their gameplay. The lightbox is a great way to turn people <i>away from</i> your product.