Not a problem...
If it's fair, but of course it's always tilted in favour of the company. I would be happy with 100GB a month for a fixed price, I pull down about 20 GB in a bad month, 2-4GB in a good one, I'm running TV episode torrents as well. If I step over my limit, then charge me 50p/GB for the extra.
As a previous poster has said, simply metering from day one at a fixed rate $1/GB or whatever will kill everything, bar email and brief lookups for critical information, however I can see one thing that will happen, people will get savvy and start blocking all the images and flash junk that wastes bandwidth, so internet advertising revenue will be down! Shame!
Hopefully market forces will "step in" and the ISPs offering un-metered services will be the real winners.
@Nexox Enigma, what the feck are you storing this 100GB/weekend downloads on, you got a rack of EMC Syms in your garage?! More to the point, what are you actually pulling down?!