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 the text Serial Number.

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

To create the 60-character long text string, append the following four strings:
 * 11-character long serial number (letters in upper case)
 * 15-character long IMEI number (without spaces)
 * 17-character long Wi-Fi MAC address (letters in lower case, including colons)
 * 17-character long Bluetooth MAC address (letters in lower case, including colons)

In short: UDID = SHA1($serial + $IMEI + $WiFiMac + $BluetoothMac)