You can find out your device’s ID using iTunes (7.7. or later). To do this, connect your device to your Mac and open iTunes. In iTunes, select your device in the ‘Devices’ section and navigate to the Summary tab. Click on the Serial Number label to reveal the Identifier field and the 40 character device ID. You then need to copy this device ID by pressing Command+C.
Basically, signing up to be a beta tester means you get our new iPhone (and iPod touch) apps via email just before they come out. You can then download and give us feedback on them.
You can choose to receive these as at the time they are submitted to apple, or once they have been accepted to be in the app store.