What this will eventually lead to...
Look, ISPs don't get their bandwidth for free. They have to pay for it.
They ALL create speed pacakges with prices based on a mixture of what they can get away with and sharing that bandwidth cost across their users, proprotionatley according to speed.
Now, if ISP A can curently give a 24MB service with no limits, then they're obviously fine. But as their subscription rates increase they're faced with the dilema of installing new equipment, more connections to the backhaul and so on. That investment isn't tiny. When they reach that point they have to ask themselves... 'Do we invest, and possibly increase out prices to cover the costs, invest and hope subscriptions increase to cover the new cost, or just try and make the current bandwidth work great for the majority of our users'.
A typical user with a 10, 20, 50MB connection primarily expects web pages to load really fast, download something from iTunes/Amazon quickly and so on. When they do this they get a good connection and maybe don't see max speed, but the DL is over and done with quickly and they feel justified in their service. Some even realise that max speed from a single site isn't possible if that site can't deliver at that speed.
Now, heavy downloaders expect xMB service to mean (xMB / 8) * 60 * 60 * 24 = Bandwidth per day.
Firstly, all domestic internet connections are contended and always have been. As such, you've no right to expect you max through put 24/7. Quite why ISPs don't make their argument that way around is beyond me, but still. Ultimatley I think its because contention management isn't very efficient.
This amounts to why no ISP should really ever say 'unlimited' downloads. Because they're all truly limited. What they should state is that 'We guarantee that you can download ((xMB / 8) * 60 * 60 * 24) / Contention ratio per day.
Just for a worked example, a 20MB connection is a max daily bandwidth of ~211GB per day. Obviously I'm ignoring protocol overhead here. If its a 50:1 contention ration then the guarantee should be: 4.2GB, and 20:1 = 10.5GB/day.
I'm not saying anyone ever will, but if they advertised like this, it would be honest, at least and should be the numbers their infrastructure is based on.
However, what is more likely is that they will move back to a pay per GB model. Which honestly if was done sensibly (ie, cheaply) wouldn't be so bad. Give everyone max speed and charge by usage. Some people pay a pittance for the 50 e-mails and 20 mp3s they download a month, others pay more for the constant bit torrenting.
Bandwidth management comes about more from the competition end of things as companies daren't try selling such products unless they all do it.