I always thought something like this would be a good idea, but it still requires the people you know to actually use it, which tends to be the sticking point.

Also, there's a significant risk that if people are shy enough to use this rather than asking someone out directly, they'll still be too shy to openly talk about it with the other person even after receiving the notice of mutual attraction. I actually had that happen with a similar feature on OKCupid (where you rate people on looks/personality, and if you have a mutually high rating, it notifies you both). I think it might be related to the game theoretical concept of common knowledge (http://www.gametheory.net/dictionary/CommonKnowledge.html) (vs. mere mutual knowledge), or something along those lines.