Last night, various Reddit users posted links to stories from ABC and other news outlets reporting that the Secret Service had been dispatched to the home of Senator Joe Biden.
That wasn't a 100% slam dunk but a pretty good indication of the pending announcement, which Obama's campaign had otherwise managed to keep in great secrecy up until that point.
Still, there's a lesson in there about “Internet time” and the fact that nothing's a secret for very long anymore in this day and age.