This is just a guess, but it is certainly plausible...
The Palin nomination is a total gimmick and won't last until Friday.
It is intended to gain news attention, create a brilliant flash, and then go away. McCain has no intention of keeping Palin on the ticket. This is just for show. Sure, it may show a lack of judgment, but time will heal that. Don't you know? McCain is a Maverick...
The second choice will show much better judgment.
After all, McCain himself didn't actually vet Palin. He only met her once or twice. Some staffer, you know, the real slacker, screwed it up.
McCain will accept Palin's heart felt apology and her 'Thanks but no Thanks' letter later this week.
The pundits will all point out the heartless bloggers and internet a--holes for pointing out her lack of experience and all the 'other problems' she has. They will be throughly beat-up for pointing out the pregnancy issues. How dare they talk about that? Jerks...
Then McCain will choose someone else. Like his actual first choice, Joe Lieberman. It will be a classic bait and switch, just like Bush did with Harriet Miers and Sam Alito.
And the sick thing? It just might work.
McCain will also get credit from the PUMA whack jobs, who will feel that McCain tried to select a woman, but the 'misogynistic' DNC just couldn't let that happen. And everyone will be too tired to even notice when McCain selects an equally terrible VP candidate...
But, this is just my guess.
Reference to history:
From wikipedia on the Miers/Alito bait switch.:
On July 1, 2005, Associate Justice Sandra Day O'Connor announced her retirement from the Supreme Court effective upon the confirmation of a successor. President George W. Bush first nominated John Roberts to the vacancy; however, when Chief Justice William Rehnquist died on September 3, Bush withdrew Roberts' nomination to fill O'Connor's seat and instead nominated Roberts to the Chief Justiceship. On October 3, President Bush nominated Harriet Miers to replace O'Connor. However, Miers withdrew her acceptance of the nomination on October 27 after encountering widespread opposition.