VP's aren't always chosen because they are ready to be president. A VP is chosen to attract, to the ticket, a specific region / constituency, or shore up a weakness. Obviously, being Alaska's "favorite son" won't secure a large block of electoral votes. So McCain is after a certain constituency because there is no evidence, that I'm aware of, that Palin shores up McCain's biggest weakness either. So what does the choice of Palin really say?
First weaknesses and strenghts.
Weakness: McCain is a self-confessed economic ignoramus and I don't see any evidence this is Palin's strenght. So at first glance, this is where the GOP ticket concedes a bunch of issues, and says, 'We don't care about jobs or the cost of living and neither should the voters". I would guess this thinking could be extended to apply to universal health care, social security/medicare, budget deficits, infrastructure, urban issues (what's Alaska's biggest city?), etc. but let's not beat a dead polar bear.
Strenght: Unless women are anti-abortion, this isn't a great pick. Sure she's a hero on the court and a beauty queen turned politician, which is a story that fans of Legally Blond would find appealing, but what is the idea with the gun-tottin' mama for VP? Do women believe it's a critical issue that they should have the opportunity kill their own dinner? If the biggest wild game in the neighborhood is pigeon that menu could get pretty boring. So that's not it.
It must be the constituency angle. Palin is a social conservative. She's a sled dog/wolf hybrid attack dog, chosen to get the right-wing "follow my rules" hardliners back in the fold. She's anti-abortion, anti-gay, pro-gun, pro-no-taxes-for-the-rich and will shout it from her pulpit. So McCain chose her to convince the right-wing "my rules and mine only" group that he won't go librul (sic) on them and they are welcome in the post-Bush world. John, shouldn't you have done that already?