If they had a $30/3GB shared I'd be all over that. My brother and I barely use 2GB as it is, and my parents certainly wouldn't use 1GB. That would bring my monthly plan down by $30/mo and my mom could get a smartphone. If they had a 5/$50 plan I might do that just to put my iPad on there too.
But there is no way that'll happen since it'll bring ARPU down dramatically. Just like charging per bit for data like a utility company would. Flat fee for connection ($5/mo) and then with usage it goes up from there. Atleast that way the bandwidth hogs would pay their fair share, but they'd lose a ton of cash from people on WiFi, etc.
What I'd really like to see is everyone switched to a $10 or $15 for 1GB plan with $10 per GB overages after that, period.
To be completely honest, they should have dropped the unlimited plans with the iPhone 4 launch. They still had iPhone exclusivity and the bleed would have been very minimal. They should have just said 'If you want to upgrade to an iPhone 4, you have to sign a new contract that has tired data only. Otherwise, you can keep your iPhone 3G/3GS on the unlimited plan until you decide to switch carriers or change to a current plan.
There's absolutely no reason to keep people grandfathered in on old plans when they go to sign a new contract. Sprint did the same thing when they went to EVDO. They just said 'If you don't have a EVDO device, you can keep your current $10/mo unlimited data and texting plan. If you want a EVDO device, then you need to sign up for the $15 Data and $15 Texting plan.