If you have downloaded iOS 5 beta, you must first activate your idevice to run it. Activating firmware means is that you have to get the UDID of your device and get it registered with the iPhone Developer Program before you can start (legally) loading beta firmware on it and you don’t face any problems while synchronizing with iTunes. Installing beta firmware on unactivated device will make it useless. Here is how you can activate iOS 5 beta 1.
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.