I remember hearing Rush Limbaugh discuss the possibility, on the phone with Fox News -- on the night of John McCain's acceptance speech at the RNC.
As I've argued immediately below, the "experience" argument hasn't really ever gained traction with voters. But if Barack did swap runnning mates, he'd implicitly be conceding that he blundered the most important professional decision he's ever had to make. That actually might be enough to make the "experience" and "preparation" arguments salient. After all, Presidents don't get do-overs.
I agree that he wouldn't keep Biden on the ticket out of loyalty. But Barack would hang in there (or make the decision to get out) out of pure self-interest. If he does try to make a Biden-Clinton swap, it's a real "Hail Mary" -- and indicates that his polling is telling him that without a desperate manuever, he's headed for almost certain defeat.