We have gone through all this a while back in the UK.
The ASA (Advertising Standards Authority) stepped in and published it's guidlines on 'Unlimited' claims.
"Unlimited" Broadband / Mobile Data Rules
* The term "unlimited" can only be used if the customer incurs no additional charge or suspension of service as a consequence of exceeding a usage threshold associated with a Fair Usage Policy (FUP), a traffic management policy or similar.
* Limitations that do affect the speed or usage of the service must also be moderate only and clearly explained in the advertisement.
So in the UK now, you can't get away with saying your service is unlimited if you charge extra after any FUP applies and you MUST explain any limitations in your advertising clearly. It must NOT be in small print.
Do AT&T mention their throttling / limits at all in any advertising ?
The ASA have the power to impose huge fines if advertising rules are breached, and have done so.