Why haven't signup confirmation emails died yet?
rodmaz: I think for some solutions the quality of data is very important, sometimes crucial. Think LinkedIn, Facebook, which are sites that count very much on the quality of the data they collect for advertisement purposes. 
Facebook for instance enforces that you enter a valid email address to belong to a network (e.g. Cornell University etc).
Maybe someone creates a solution, a 3rd-party app which could do the trick (maybe even use Twitter for that). Ideas?? :)