Find UDID of iDevice to Activate iOS 5
How you can find the UDID of your iPhone, iPad and iPod touch for iOS 5 activation. UDID of your device must be registered with the Apple Developer Program before beta firmware installation on it. Because iOS 5 is in beta phase right now, you need to register the device UDID, and here is how you can find UDID of your iDevice.
Step 1: Plug in your iPhone, iPad or iPod touch to the computer
Step 2: Launch iTunes.
Step 3: In iTunes, select your device by clicking on its name in the left sidebar.
Step 4: Hover mouse to where it says Serial Number and click on it.
Serial Number will disappear, and in its place, Identifier (UDID) will appear. This is your device UDID. This is your device UDID which you must get registered before you install beta firmware on your iPhone, iPad or iPod touch. If you do not get your UDID registered, your device will run the beta firmware, however, it will not be able to sync with iTunes, until and unless you get it registered.