Clients often ask for UDIDs to add to provisioning profiles for iPhone app testing. We’ve always used one of the many UDID apps that are available for free in the App Store, but have found that those don’t work well any longer.
The easiest way to obtain the UDID off of an iPhone or iPad is to plug the device into your computer and launch iTunes. Within the Summary tab of the device settings in iTunes you’ll see the “Serial Number”. If you click that it will change to show the UDID – right-click (PC) or control-click (mac) will allow you to copy the UDID.
One key is to make sure that it isn’t prefixed with “FFFFFFFF” – that is an easy way to identify an invalid UDID.
NOTE: There are other ways to get the UDID, but we’ve found this way to be the easiest.