iBook and Powerbook 12 inch: 106 dpi (1024x768)
iBook 14 inch: 91 dpi (1024x768)
Powerbook 15 inch: 101 dpi (1280x854)
Powerbook 17 inch/iMac 17 inch: 100 dpi (1440x900)
Cinema Display 20 inch/iMac 20 inch: 99 dpi (1680x1050)
Cinema Display 23 inch: 98 dpi (1920x1200)
Cinema Display 30 inch: 100dpi (2560x1600)

Very consistant DPI numbers across Apple's LCD lineup except the 14 inch iBook.