Obama says he's made his choice and he's not telling.

In other words, na na a boo-boo I know something you don't know. “I did say that I’ve made the selection and that’s all you’re going to get,” He also said, "Wouldn't you like to know?" to reporters in Virginia when asked who it was.

I don't know, but it seems like aggravating the press might not be the best strategy. Also maybe it's comments like those that make it easy to paint him as a egomanical know-it-all elitest snob. Either that or a five year old. It can be hard to tell the difference sometimes.
http://news.yahoo.com/s/ap/20080822/ap_on_el_pr/veepstakes