Katy Perry, Pepsi and MTV are doing some sort of convoluted hashtag-related unlocking system thing that sounds like it could actually be quite good.
Basically people have to tweet #katynow in order to "unlock exclusive content" which includes "song titles, lyrics and sneak peeks" from 'Prism'.
Fans can then apparently also decide which songs Perry releases ahead of the album, although that might be pushing it a bit let's be honest.
Anyway, all the info is here if you're interested.