Anton,

You're right on the money.  Where AT&T (and other providers) has been required to maintain a black list, it is my understanding they work with the IMEI DB which is accessible to all GSMA members (AT&T being a founding member of the GSM Association, of course, has access to this).

The IMEI DB is a database of blacklisted phones and various vendors can update their own blacklists, and update this ensuring phones reported as stolen do not do anything more than dial 911 (or whatever the local emergency call number is).

A stolen phone generates non-subsidized profit.  Since, at least in the US, most cell phones are subsidized based on the customer agreeing to a multi-year service contract, a STOLEN phone that is activated on a cellular network is pretty much "pure profit" for the cellular provider.  The cellular provider doesn't have to 'front' the cost of the phone and hope the contract is maintained long enough to pay equipment costs of the handset.  Nope, a stolen phone generates profit from the moment the person posessing the stolen phone activates a contract.

Why in god's name would ANY cellular provider ever voluntarily blacklist any phone?