As Elephant Watcher observed when Huntsman made his announcement, it's possible that Huntsman feels there's a benefit to running even if he doesn't win. If, say, Mitt Romney wins the nomination and President Obama is reelected in 2012, there will be an opening for a Republican to run in 2016 with no incumbent opponent from either party. Since the Republican Party has a history of nominating the runner-up from the previous nomination contest (or at least someone who has run before), it could be to Huntsman's benefit in 2016 if he ran in 2012.
Another theory is that Jon Huntsman, with his considerable diplomatic and foreign policy experience, is actually running for the position of secretary of state. If one of his opponents wins the Republican nomination and defeats Obama, that new Republican president could appoint Huntsman as his secretary of state. The idea goes that by running for the nomination and failing, Huntsman still raises his visibility and gets many opportunities to advertise his diplomatic credentials.
The third theory is the most obvious one, that Huntsman intends to win the presidency in 2012. The odds may be against him, but he does have a path to victory: through Romney. If Huntsman can topple Romney in New Hampshire, he will replace Romney and have a shot at the nomination. Anyone who wins the Republican nomination, especially if he is highly electable, stands a good chance at winning the general election.
It may be possible to deduce from Huntsman's behavior which of the jobs he's after. Each of these theories requires a different scenario. Let's start by looking at the first scenario, where Huntsman intends to run for president in 2016. This requires Obama to win reelection. Thus, we would expect Huntsman to spend a lot less time attacking Obama. Normally a candidate spends a lot of time criticizing the incumbent of the other party, but in this scenario Huntsman actually wants Obama to win reelection. Huntsman may even want to spend more time attacking his Republican opponents, though he would want to avoid making permanent enemies.
If Huntsman wants to be secretary of state, he needs for Obama to lose. Thus, he may be more willing to attack Obama. He would be a lot less willing to attack the Republican whom he thinks will win the nomination; after all, that would be the person who wins the general election and appoints Huntsman secretary of state. The biggest hint of all would be if Huntsman spends a lot of his campaign talking about foreign policy. As we've observed numerous times, the 2012 election will be about the economy. Huntsman has a conservative economic record, so it's not as though he needs to avoid that topic in lieu of discussing foreign policy. But if Huntsman wants to be appointed secretary of state, it would make more sense for him to advertise his diplomatic and foreign policy credentials.
Finally, if Huntsman truly intends to win the presidency in 2012, he will need to attack Romney and Obama. He needs both of them to lose for himself to win. And he will focus more on economic policy, since that will be the issue voters look at when picking their president.