Why has Andy Coulson decided to resign today?

c) The crown prosecution service is now reviewing all the data the met (who apparently think that breaking the data protection laws by newspapers is only worthy of prosecution when the victim is connected to royalty) collected so in a month or three it really would be embarring if Coulson was still working for the prime minister when he's called up before the beak. So that's why he's resigned, as he's on the verge of being caught for being a spy voyeur, and he's resigned today in the hope people wont notice.