The finalists are Tim Howard, Clint Dempsey and Landon Donovan.
For my money, this should be Howard's all the way. If form over the past year is important, Howard should win hands down. He helped his club get to UEFA Cup (although shambolic defending and lack of scoring got them bounced). Howard has had four shutouts in the U.S.'s last six matches (Brad Guzan had two against Barbados) including the performance of the year agaisnt Argentina (a performance worth the prize on its own). Even when he doesn't keep a clean sheet, Howard's performance rarely falters and most goals are not his fault and he saves the back line more than he lets them down.
Dempsey has a special place in my heart as a Fulham supporter. But to be frank, he is out of favor and out of form with his club. Fulham are playing well (not getting results lately, but playing well) and honestly Dempsey is not part of the package. He has been good enough to dress for every game despite not getting a lot of minutes, so that says something, but the measure is playing time. Dempsey has been finding his form for the U.S. of late, but over the past year, he has been less than his usual successful self. I think this nomination is more a factor of regular U.S. appearances rather than quality on the pitch.
Landon Donovan has been electric for the L.A. Galaxy and without his goal scoring they clearly would suck even more than they do. However, despite his prowess at putting the ball into the net, you can't get past the general form of his club. Add to that the fact that Donovan has largely been absent for the U.S. He has flashes of flair, and to be fair, Bob Bradly may not be using Donovan to his best talents, i.e. running at defenses down route one, but to be fair, he has not been a factor in the run of play of late for the U.S.
So for his performance for club and country--Tim Howard--hands down.