After his first start against the Red Sox, multiple teams have their eyes on Harden including the Red Sox, Mets, Tigers, Yankees, and many others will most likely join the race.
It is unlikely that the Mets will be able to secure the ace because of their now depleted minor league system. The most they can muster would be Jon Niese, Fernando Martinez, Eddie Kunz, and Mike Carp. However, that may not be enough, and may be too much of their future to mortgage.
The Sox and Yanks are much more likely to be able to make a serious bid at the star. The Sox have got a stocked minor league system, and multiple expendable prospects. The Yankees may be willing to give up one of their big three young pitchers, Joba Chamberlain, Phil Hughes, and Ian Kennedy, and another good prospect.
Of course each team may want to see Harden pitch a few more times to see if he has fully recovered from injury. Either way Rich Harden may be in a different uniform as the A's start to rebuild.