In short, the GOP primaries could decide who gets to be the Landon/Goldwater/McGovern/Mondale of 2012 if most things go right for Obama -- no foreign-policy blowups, no personal scandal, and some well-received reforms of the economic and political systems.
I still remember quite clearly back in 1992 a SNL skit parodying the Democratic primaries - who gets to be the guy to lose to George H.W. Bush in the fall? IN the skit, everyone was contending to not be the nominee.
Things change, and it is unwise to count your chickens too early...
This seems so obvious that it can't be original: there is no "typical Presidential election" in American history. Every one of them is different (except arguably 1952 and 1956). Every President is different, and the conditions under which every President wins is different.
I can't predict that there won't be a strong third-Party challenge in 2012-- yet.
Funny enough, 2008 was the year that history dictates that a third party challenger would have emerged.
The two parties nominated the least obviously partisan, most electable candidates possible, squeezing out Bloomberg or anyone else's chances.
Edwards vs. Huckabee or Romney would have seen a Bloomberg candidacy, not sure about the other match-ups.