We have a family plan that gives 700 min/month plus unlimited data and 200 SMS each. I will get a new iPhone 3G for myself, give my old 2G iPhone to my wife and ditch the Nokia. Right now I pay $70/month + 2 x $20 for the family plan. Total $110 + tax.
The new plan will be $70/month + $30 for 3G + $20 for 2G, including 200 SMS for use on the 2G only. Individual text messages on the 3G are 20¢ each, in or out. Total is $120 + tax. There is the $18 charge to upgrade the 2G to 3G, no charge to replace Nokia with 2G.
I talked with an ATT rep today.
I had a plan with Sprint that allowed bluetooth tethering for $15/month. including phone internet use. That plan has been discontinued, but I used it until I got the iPhones. The on-phone internet was pretty much useless, but the tethering worked well on trips cross country.