(this does piss me off)
Yes, there’s a limit to the amount of data that an ISP should allow it’s users to transfer in a given month. That’s the speed of the connection (megabytes per second) times the number of seconds in a month.
If they don’t want as much data flowing through their networks, lower the speeds users can achieve. Don’t wow us with the speed, only to tell us that we can’t use it every single second of the day.
Update: Cory Doctorow describes this problem (though doesn’t offer the suggestion) on TWiT 124 at about 52 minutes into it. The suggestion is, lower the speed when you’ve got too many users in any location, but make sure we know that that’s the true limit of the connection.