what does at&t giving up AWS spectrum to T-mobile mean?

I was reading this article on the verge http://www.theverge.com/2012/1/24/2731330/t-mobile-new-spectrum-map-att#89811679 and realized I really don't understand whats going on.

Originally I had thought that AT&T would be letting t-mobile customers roam onto their 1900mhz (t-mo iphone users rejoice) but if they're giving up AWS specifically, does that mean in these markets AT&T owned the license for 1700mhz and T-mobile customers didn't get 3G? The map in that article shows places where AT&T is giving up 10mhz and 20mhz specifically, but I don't understand how that pertains to AWS either. Does this mean frequencies up to 20mhz adjacent to 1700mhz would be empty so there's less Interference?

What real world difference does this make to customers of either company?