I have no idea, but I'd guess it points to a bigger name rather than a smaller one. (I think Jonathan Cohn made a version of this point earlier in the week, though I can't remember if he did it online or during one of our many rounds of internal speculation.)
You can let the suspense build and build if you've got a Hillary or a Gore socked away somewhere. Possibly a Biden or a Webb (or some unorthodox pick like a general or a Republican). But you'd better not come with Jack Reed or Evan Bayh after toying with people for over a week.
Update: Mark Blumenthal has a great theory:
In retrospect, I think the reasons for the timing seem obvious to me. The slow drip of "news" is entirely consistent with maximizing response to their "Be the First To Know" email/text message campaign.
So Team Obama vacuums up names, e-mail addresses, and cell phone numbers while we wait, information that'll come in handy down the road. Genius. (Though I still say you can't then turn around and pick Jack Reed...)
Relatedly, I'll bet a higher-than-usual percentage of the e-mails the campaign sends out are getting read this week. Who's gonna delete an unread e-mail from the Obama campaign knowing it could include the veep's name...
--Noam Scheiber