Well seeing that alot of people are wanting Dev's to register their UDID for 4.0 I thought of making this so people stop asking what's the UDID and how do we find it questions. What's a UDID? The UDID is like a personal ID made up of 40 letters/numbers. Every device has a custom one that is registered to Apple. What can I do with my UDID? For the 4.0 OS case Dev's need your UDID to register that you are allowed to use the firmware to test out their apps on 4.0. If a Dev does not register you, upgrading to 4.0 will give you a error in itunes. Well how do I find my UDID? The easiest and most simpliest way to do this is to connect your device, open iTunes, and go to your device's control panel in iTunes. Now where it says "Serial Number", press on it and then the words will change to "Identifier (UDID)". Now next to it is your UDID. Please Register or Log in to view images Please Register or Log in to view images Now I have my UDID, now what? For 4.0 beta testing you need to give a Dev that is in the development program your UDID so he can register you to be able to use 4.0. Another way of using this is that some apps, even before they are in the app store can recieve beta testing just like the 4.0 OS, you'll need to give the Dev your UDID so he can register your device to be able to use the app.