I got the same postcard and, like you, assumed that it was not to my benefit.
However, it appears that this is the result of a lawsuit brought against AT&T (long distance) over incorrectly billing customers a city tax.
Check out
http://www.oclsettlement.com for a list of cash and prizes you are entitled to.
It would have been nice if they had put a URL on the postcard to provide more information. You could call the 800 number on the card, I suppose. But then, that may give them the right to have telemarketers call you with money-saving offers for your long distance service.
This is the closest thing I could find on their site:
http://www.corp.att.com/attsettle/