A tipster has provided AppleInsider with information that Apple may have been planning a higher-density display for the 17" Powerbook, increasing the current resolution from 1440x900 up to 1920 x 1200. This change would effectively double the screen resolution, resulting in a perceived larger desktop area for the user without an increase in physical size.
There is not a Powerbook currently offered with this higher-density screen, either as a standard configuration or as a Build-To-Order model.
[Edit] Original source of the information came from macosXrumors.com.