NBC's Tim Russert reported that the Obama campaign will be able to claim "a mathematical lock on the nomination" by June 4th.
Russert said, "Five months after the Iowa caucuses, Barack Obama needs just about 40 delegates to secure the nomination. 86 delegates that are available in Puerto Rico on stunned, Montana and South Dakota on Tuesday. He will get those. Now, enter Michigan and Florida. There's a meeting this coming Saturday which will have a deal seating some of the delegates from Michigan and Florida, which means Obama will have to pick up probably another 40 delegates. Of the 200 superdelegates left, he only needs 40 of them. They have identified them. The Obama campaign will claim a mathematical lock on the nomination one week from tonight, wednesday morning at the latest."
This video is from NBC's Nightly News, broadcast May 27, 2008.