I decided to compile all iOS devices specs here so we can get a better glimpse on their hardware and what we can expect from them.
This covers iPhones, iPod Touches and iPad.
iPhone/IPhone 3G – 412 MHz / 128MB / 320×480
iPhone 3GS – 600 MHz / 256MB / 320×480
iPhone 4 -~800MHz / 512MB / 640×960
iPod Touch 1G – 412 MHz / 128MB / 320×480
iPod Touch 2G – 532 MHz / 128MB / 320×480
iPod Touch 3G – 600 MHz / 256MB / 320×480
iPod Touch 4G – 800 MHz / 256MB / 640×960
iPad – 1 GHz / 256MB /1024×768
I think this data is correct (Wikipedia) but let me know if some is wrong.
As you can see there’s some great deal of difference in CPU clocks on some models. So watch out if you want to target all generations since some games can be run slow on older devices.