320 x 480 (iPhone & iPod Touch)
640 x 480 (G1 phone -- use the free application "Wallpaper Set and Save" to avoid the OS's JPEG recompression artifacts)
800 x 600
1024 x 768
1074 x 768
1280 x 960
1280 x 800
1280 x 1024
1400 x 1050
1440 x 900
1600 x 1200
1680 x 1050
1920 x 1200