I definitely had this problem when I was an AT&T customer with an "unlimited" data plan, so color me unsurprised:

The Federal Trade Commission has sued AT&T for promising unlimited data to wireless customers and then throttling their speeds by as much as 90 percent, the FTC announced Tuesday.

All major carriers throttle certain customers during times and places of congestion, as we've reported previously. AT&T seems to have earned the FTC's wrath by throttling customers regardless of whether they were trying to use their phones in congested areas, however. As we've also written, AT&T was throttling unlimited subscribers regardless of network conditions until July, when it changed its policy. Throttling was enforced once users hit 3GB or 5GB of data per month.

The FTC's lawsuit in US District Court in San Francisco alleges that AT&T hit unlimited data customers with an "unfair mobile data throttling program" and that AT&T committed a "deceptive failure to disclose [the] mobile data throttling program."

AT&T denied the allegations, saying that its practices are similar to those of other carriers and that it has been "completely transparent with customers since the very beginning."