This story is not definitive evidence but unless people are lying it's hard to think of an alternate explanation.
One thing that makes me pretty suspicious, though: This here logo is awfully spartan and bears no resemblance to the sleek and very popular aesthetic of Obama's other campaign materials....
Update: Ambinder pronounces the sticker "too ugly" to be real, and notes that it lacks a union label. Arghh, this is like exit polls all over again.
Update II: So much for that: It's not Bayh or Kaine. (That leaves one name of the three listed by the Times a few days ago, also starting with B....)