The APIs are in place as of 10.4 to scale things properly, meaning it will be a 10.5 feature most likely. Also, Apple's displays have been around 100dpi for a while, with the MacBooks moving closer to 110. There was rumors of a "PowerBook HD" around the last G4 revision, due to Apple documentation talking about a higher resolution screen option of 1920x1200. For whatever reason it was pulled, and aparently didn't make it back for the 17 inch MacBook.