Even if there were 150 dpi screens there wouldn't be any reason for you to tell the OS that. It would be able to tell from the screen automatically and rescale the UI elements to match it.
fryke said:Captain Code: "would" is the important word here.
Elliotjnewman said:But there could be multiple images, say, apple could design the whole interface at a huge res, and then just have it scaled (using nice resamples) down many times.