A simple Google Search of "Biden Dumped for Clinton" gives a stream of blog buzz regarding Barry Hussein's major flop in selecting Joey "Hair-Plug" as his Vice Presidential Running Mate - beginning around the time of Joe Biden's "Hillary would have made a better choice" comment.
Of course Biden is the Gaffe-master, but is he really setting the stage for a VP switch, as suggested by the blogging world? If so, what would that look like, and what legalities would have to take place? Surely an October 5th bait and switch would not leave ample time to change ballots in the 50 states plus territories voting...
The DNC's bylaws simply describe one of the committee's responsibilities as "filling vacancies in the nominations for the office of President and Vice President." (See Page 7 of this document [PDF].) The bylaws do not, however, explain how the DNC would go about making its decision or how all ~400 members of the committee would be allowed to vote.
The most recent case was in 1972, when Thomas Eagleton withdrew from the ticket under George McGovern after disclosure of previous shock therapy brought into question the seriousness of his depression. In this case, a "mini-convention" was convened on Aug 8th (as the conventions of that election were held MUCH earlier than the 2008 election), 275 DNC members attended, and had to formally elect a new candidate for VP - in that case, Shriver.
Simply put, Obama cannot "simply" replace his VP pick... though Biden may withdraw of his own accord - but it may be political suicide for him... it is notable, however, that he is running a re-election campaign for the senate in conjunction to the Vice Presidential bid - as allowed by Deleware state law. A withdrawal for severe health reasons may hinder the Obama candidacy, but it may bring into question his ability to hold a Senate seat for an additional 6 years.
That being said, the DNC would have to officially reconvene in some extent to reconfirm a new Vice Presidential nominee.
Politically, such a move by the Democratic Party would be seen as a last-ditch move on the part of the Democrats. It would demoralize the Democratic Party. Although a decent number would welcome an Obama-Clinton ticket, an equal number would see it for what it is: sheer desperation.
The Republican backlash across the nation would result in an overwhelming Republican turnout and Republican landslide not seen since Nixon defeated the McGovern/Shriver ticket in 1972.