Kryptonite

Pro Basketball Talk has a piece on the players in the league who can slow Dwight Howard down. 

Kendrick Perkins The first true Superman stopper (or slower downer?) has to be Kendrick Perkins. Having the size to match up with the mammoth is a huge advantage in slowing Dwight down, however, having that size alone doesn’t get the job done. Perkins is an exceptional defender especially against Dwight. He pushes Dwight away from the basket with his physical play and dares Dwight to beat him with his finesse offensive moves, or lack thereof. Having a defender like Perkins on Dwight allows everyone else to match up with their men without worrying about the help defense. Just hope that you don’t have to deal with foul trouble.

The key to Perk's defense on Howard is that he can do it with single coverage.  With Perk, the team doesn't need to double team Howard and that is huge when you have other shooters to guard like Kobe, Nash, Artest and Gasol.   They list only 3 players who have had success in slowing down Howard in the past and Perk is at the top.  With Howard's move to one of the Thunder's top rivals, Perk becomes even more important to the team this season.