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. The UDID can actually be changed if your device's MAC address (WiFi or Bluetooth) changes, which isn't likely but still possible.

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 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)
 * 15-character long IMEI number (without spaces); empty string for iPod touch and Wi-Fi model iPads (on older devices) OR on newer devices 13-character long "ECID" in decimal (like shown in Xcode)
 * 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
For older devices: UDID = SHA1($serial + $IMEI + $WiFiMac + $BluetoothMac)

On the Verizon iPhone 4 and newer it is instead computed with this formula: UDID = SHA1($serial + $ECID + $WiFiMac + $BluetoothMac)