If you use ad-hoc distribution for testing or for letting your clients review applications, you need to have the UUID of any phone that you want to be able to install the application on. For people who have the developer tools installed, it's easy - you just copy it out of the Organizer window in Xcode. For people who don't, however, iTunes will show you the UUID, but it won't let you copy it to the clipboard, meaning somebody has to manually type it in, which is error prone and sucks. I've had many problems with mis-typed UUIDs from clients.
There is an alternative, at least if your users are on the Mac. The System Profiler application located in /Applications/Utilities/ will show you the identifier for any iPhone currently connected to the computer. Just select USB from the left-hand column, then find the entry for iPhone in the upper right, and select it. Once you do that, it will bring up information about the iPhone. The field labeled "Serial Number:" is the phone's UUID, and it can be copied to the clipboard. You can see it in the screenshot below (though I've blurred out my UUID):
Edit:Apparently, it is possible to copy the UUID from iTunes, though it's non-obvious how to do it. If you click the Serial Number to show the UUID, then double-click the actual UUID value, it won't highlight, but you can actually press ⌘C to copy the value to the clipboard. Not very intuitive, but it works. I think System Profiler is actually easier to explain, since the highlight behavior is more in-line with the way the Mac generally works.