What is a UDID? No it's not something funky or a four letter short-hand acronym for use on Twitter or your favorite social networks. UDID stands for "Unique Device IDentifier" which is a 40-character, unique set of numbers and letters identifying your iPhone hardware. Apple has set up standards to ensure that the UDID usage doesn't violate user privacy.
 
Ah ha, but not everyone works ethically and so it appears from a study conducted by a researcher from Bucknell University where he found that private user data may be transmitted along with the UDID.

The study found that of the 57 top Apple Apps, almost 70 percent of the apps sent out the UDID and many apps dropped cookies on the phone that don't expire for years. This means that even old discarded phones can be linked to using the UDID information.

Unfortunately the UDID cannot be switched off so beware users, your phone is watching you and if your UDID is being transmitted to third party servers, who knows what else is being transmitted with it?