Project LovePlus, an upcoming Japanese dating simulation from Konami, will use the Nintendo 3DS' camera to recognize its main user's face and make sure the virtual girlfriend in the game doesn't stray to any of his backstabbin' friends.
Famitsu says that, should someone else pick up Project LovePlus, they'll encounter the "Boyfriend Lock," which will make the girl unresponsive to anyone who isn't her man. She'll react with confusion and questions like "Who are you?"
This is a crude first step in the world of dating sims. What I think needs to happen next is the girl immediately opens up about her relationship problems to the first guy who isn't her boyfriend. Then, after a long period of pretending to listen to (and care about) her problems, the game presents an opportunity to make your move. Or there could be a "Just Friends" mode, in which the game determines the girl is seeing an emotional, distant lout, and relies on the second user to supply all the emotional support she's not getting, with no affection.