Artifacts of Polling
As I understand it, the so-called "15-point bounce" is generally associated with the period between naming a running mate and accepting the nomination at the convention, so it's difficult to say how much of that we're seeing in this particular poll. I think the important thing to recognize is that these numbers are bound to fluctuate between now and November, sometimes dramatically. This just happens to be one such surge, so -- as I think hella is pointing out -- it would be a mistake to read too much into it.
I think some of the other results of this poll are actually more interesting. Cheney, for example, gets a generally poor approval rating here. But the poll also shows that "many voters still don't know who Edwards is... Eleven percent say they have never heard of Edwards."
Ref: MSNBC - Poll: Kerry gets a bounce from Edwards pick