Can't say I blame them, somebody has to pay for the traffic and if there are a few outliers that suck up a very disproportionate amount of capacity then it makes sense. It could become a PIA if they get too cute w/ limits and mulitiple tiered pay systems...they may get a backlash if they went to something like that with too many tiers where people have to constantly think about how much time and how they are using the internet. Kind of like the old minute plans in the early days.
By the way, does anyone know a way to tell how much your monthly, daily, etc usage is? Are there any download programs that can track and sum that up? I actually work from home and have some programs open all the time that are constantly receiving and sending information, but I don't have a clue how much data it really adds up to.
EDIT: That was way too easy. Here is a recent article re: this issue and a download to track usage.
How to track your bandwidth usage