Existing users in the trial area will receive a usage amount of 150G bytes per month.

The caps appear to be high enough that the average user is unlikely to hit them, said Yankee Group analyst Andy Castonguay. He applauded AT&T giving customers some information and warning. But he still thinks consumers may feel they are getting less for their money.

It's in the nature of the Internet for consumers to use different amounts of bandwidth from month to month, as they try out new things, Castonguay said. A cap doesn't suit that usage, he said. It might be better for carriers to charge extra for a higher quality of service, such as always having one's own packets at the front of the network queue, or let subscribers choose to get large downloads delivered the next morning, he said. That would let the service provider put its network to work during less busy hours.

The upside may eventually be more options for consumers, Castonguay said.

"It's likely that hungrier carriers out there ... will seize this and other attempts at capping as an option to do more interesting things," Castonguay said.