Wireless bill shock is one of those know-it-when-you-see-it things. It happens when you get socked with charges that are hundreds or even of thousands of dollars higher than your normal monthly bill. See some local cases here ($17,500 to watch a movie in the Dominican Republic) and here (unexpected data roaming charges in Europe).
The FCC says bill shock has happened to about 1 in 6 wireless customers - about 30 million people. The agency defines it as "a sudden and unexpected increase in monthly wireless bills" when consumers don't realize they've blown past plan limits for voice, data and text, or get hit with unexpected international roaming charges.
At least for now, the FCC is taking a voluntary approach to addressing the problem, which coincidentally makes somebody a lot of money - though it's not entirely clear how the U.S. carriers share in proceeds collected on behalf of overseas roaming partners.
The voluntary approach relies on alerts to people at risk of overage charges, and carries an October 2012 deadline. To encourage quicker compliance, the agency is keeping public track of how the carriers are doing. So far, only T-Mobile has hit the trifecta, offering alerts for voice, data, and international-roaming overages.
You can find the chart, which includes links to the carriers' policies, by clicking here.