All Things D receives a statement from Apple:
The FBI has not requested this information from Apple, nor have we provided it to the FBI or any organization. Additionally, with iOS 6 we introduced a new set of APIs meant to replace the use of the UDID and will soon be banning the use of UDID.
Meanwhile the FBI also disavowed any involvement with the reported leak/theft of UDIDs:
The FBI is aware of published reports alleging that an FBI laptop was compromised and private data regarding Apple UDIDs was exposed. At this time, there is no evidence indicating that an FBI laptop was compromised or that the FBI either sought or obtained this data.