Biden: Was chosing him a mistake?

I've heard some speculation that Barack Obama might at some point re-think his choice of Senator Joe Biden as his running mate and select someone else, more palatable to the electorate.

Rather than bolster his numbers, like Sarah Palin's selection seems to have done for John McCain's Presidential bid, Obama'schoice of Biden appears to be bogging him down. Biden's appearances have been less than stellar and may have led to Obama's recent weakening poll numbers.

Here's a question: If Obama decides to dump Biden in favor of another candidate, can he do it? Is there any sort of rule or protocol preventing this? Is there even a precedent of such a change having occurred before?

Interesting...