Apple does provide this in Activity Monitor. Click the "Network" tab and you'll see "Data received:" and "Data sent:". It tally's since the last reboot.

It would be nice to have a history feature though. Plus, there's no easy way to ask "How much bandwidth do I have left this payment period before Comcast starts billing me for going over my monthly limit?"

You'd have to tally total bandwidth, and then associate it with an internet connection. You could track internet connections via DNS/public-IP info, and possibly MAC address for WAPs. If you're a laptop user jumping from WAP-to-WAP, it would be nice to know when you're online at home, or in a Starbucks, and differentiate total bandwidth used for each.