Some times you may need another way to find the UDID of the device as I did. (Besides iTunes device's summary page)
Recently I try to install iOS Beta version on my iPad, but I forgot to register the device with Apple Developer portal, so the iTunes device's summary page only show me the message to register the device with the portal.
Here is how I found the device's UDID without iTunes:
1.) Open 'System Profiler', under "Application > Utilities > System Profiler"
2.) Under the 'Contents' on your left, select "USB"
3.) Then select the device (iPad/iPhone/iPod)
4.) The "Serial Number"is the UDID of the device.
Hope this help.
