Although despised by senatorial colleagues (not one endorsed him in 2000), McCain is a Westerner, a war hero and a Republican who oftentimes takes moderate stances. Huntsman's affection for him is understandable, including the fact that a McCain administration offers the best opportunities for the youngish governor.
Some detractors are accusing Huntsman of political gamesmanship because his father Jon Huntsman is supporting Romney, and Huntsman family members are major bipartisan donors on the federal level. Their contention is that Huntsman has "all the bases covered" no matter who wins in 2008.
McCain has more foreign affairs experience at a time of war and international turmoil. As frontrunner, McCain also has a better chance of winning. But Romney would make a terrific president, and his campaign is going better than anyone expected. Huntsman's move this early was a big surprise, and most likely a personal blow to Romney, although not a big factor overall. Certainly, no deal was made for any job. If Huntsman is freshly re-elected in 2008, he couldn't very well leave the governorship to hop aboard a new McCain administration. But over a couple of terms, a lot of turnover occurs in a president's Cabinet, and it won't hurt Huntsman to have a solid relationship with McCain. My concern is that I've heard too often from Utah's congressional delegation that McCain is not a team player, is a prima donna, and doesn't have the temperament to be a good president.
For more background on this issue, including my own take when the news broke see here.