I had heard of The Killers (!!!), Paramore, Sia, Cee Lo Green, Linkin Park, Bruno Mars, OK Go, Vampire Weekend, Christina Perri, Florence and the Machine, The Black Keys, and I had heard the word "Muse" before Twilight.
The soundtracks got me interested in Iron & Wine, Sleeping At Last, Aqualung & Lucy something, The Dead Weather, Bombay Bicycle Club, Lykke Li, and Anya Marina.
Quite a few actually. I think I had liked Cee Lo Green, Bruno Mars, and Christina Perri (a thousand years is possibly one of my paborito songs at the moment!). Twilight got me interested in Paramore, the Killers, and a few other Florence and the Machine songs :)
Not really, because I don't listen to that kind of music that often to begin with. I do listen to Linkin Park, Christina Perri, and Paramore sometimes, but I had already heard of them before the soundtracks.