UDID

UDID is the short form for Unique Device Identifier. It is a 40-character long hex value (20 bytes). The UDID is being used by developers to register devices for testing their Apps. Only on registered devices developers can run Apps that are not in Apple's AppStore yet. Also, beta firmware can be installed only on registered devices. Developers currently have 100 UDIDs they can register. Instead of calculating the value, it can be read from iTunes, when the device is connected, by clicking on Serial Number.

Calculation
The UDID is being calculated like this:
 * Create a 60-character long or 59-character long (on newer devices) text string (see below)
 * Calculate the SHA1 hash of the text string. The result is the UDID.

To create the text string, append the following four strings:
 * 11-character long or 12-character long (on newer devices) serial number (exactly like it is written in the Settings app)
 * one of these:
 * (on older devices) 15-character long IMEI number (without spaces); empty string for iPod touch and Wi-Fi model iPads
 * (on newer devices) 13-character long ECID in decimal, no leading zeroes
 * 17-character long Wi-Fi MAC address (letters in lower case, including colons); for the iPod touch first generation use "00:00:00:00:00:00"
 * 17-character long Bluetooth MAC address (letters in lower case, including colons)

In short
On the Verizon iPhone 4 and newer: UDID = SHA1(serial + ECID + wifiMac + bluetoothMac)

All else: UDID = SHA1(serial + IMEI + wifiMac + bluetoothMac)

Changing UDID
The UDID can technically be changed and back again if you run this command followed by a DFU restore or else iTunes cannot read the device (corruption?). Keep a backup, you can always revert the change with the original MAC. As shown on an iPod touch 4G. Command:

nvram wifiaddr=ma:ca:dd:re:as

- User notice: This will change your UDID, anything UDID related will need to be re-"logined", Apple related apps will always need to be logined.