If you do something a bit foolhardy like installing iOS 5 Beta 1 on a device that's not yet registered as a development device, you can rapidly end up in a catch 22 situation — you can't activate the device because it's not registered on the iOS Provisioning Portal, and you can't easily view the device UDID that you need to enter on the portal because iTunes won't let you view the details of an unregistered iOS 5 device.
Fortunately, there's a way around it. If you go to → About This Mac → More Info → USB, you should see an iPad (or iPhone) device listed under one of the trees. On my i7 iMac, it's under USB High-Speed Bus → Hub.
From there, copy and paste the Serial Number field from the detail pane and use that as the UDID you need to enter on the Provisioning Portal.

