I see Dirk as a power forward. Just because he's a jump shooting power forward, it doesn't make him a shooting guard. And especially over the last few years, he's more of a power forward than a perimeter wing. He does more midpost or high post back downs. And he shoots far fewer three pointers than he used to. He only shoots about 2 a game. Similar to Chris Webber or Karl Malone later in his career, he's a jump shooting power forward, but a power forward nonetheless who plays the high post on offense like many power forwards do. You're not going to see Dirk consistently defend 2-guards in the league.
Tim Duncan plays center. He's the one true low post threat on the Spurs. The Spurs don't post up Antonio McDyess or Matt Bonner. Dice and Bonner are not centers. Duncan plays center on offense and anchors the defense as the center.
To suggest Dirk is a shooting guard simply because you're butt hurt that Tim Duncan is actually a center when he and the Spurs want to call him a "power forward" is being pretty sensitive. Dirk is a power forward. Tim Duncan is a center. Or are Antonio McDyess and Matt Bonner the Spurs' centers? Well, if it makes Timmy and Spurs fans happy and that's how the Spurs are going to call him in their media guide, it must be true.