Adam Nagourney and Jeff Zeleny write in today's New York Times about the end-game of Barack Obama's search for a running mate. They don't purport to know the exact timing of the announcement, but they indicate it could be as early as tomorrow.
According to their sources, the Obama campaign is planning for the ticket to tour around a bit, before arriving in Denver for next week's convention. That would necessitate an announcement soon.
Nagourney and Zeleny mention a three-person short list: Senators Evan Bayh of Indiana and Joe Biden of Delaware, and Gov. Tim Kaine of Virginia.