Nate Silver urges everyone be patient:
…our research has concluded that there typically is not any bounce until the third day of the convention. As such, this polling tells us nothing at all about the convention so far, and it probably won’t tell us a whole lot until at least Thursday or Friday.
It might tell us something about Joe Biden. I tend to agree with the conventional wisdom that there was liable to be a bit of a near-term backlash whenever Obama announced his VP choice, provided that the VP was not Hillary Clinton. The key phrase in there, however, is "near-term". If Hillary is able to rally her supporters to the Obama-Biden ticket…there could still be a latent/lagged VP bounce for Obama that gets rolled up into his convention bounce.
The Dish 