My first $2200 Phone Bill I bought an ATT Tilt online with a pda personal plan (unlimited @ 29.99/month on 1/23). Apparently, the billing system does not talk to the ecommerce system, so the Tilt was placed on a Medianet 1MB plan.
After about 30 days of RSS, checking 3 email accounts every 5 minutes, downloading some CABs and watching MobiTV I got a phone bill for $2200.
So not only do they want to charge you a huge amount of money for data, they do not want to protect you either, like sending you an SMS when charges exceed a certain amount.
The most aggravating part of the experience was having to wait for a $2100 credit to be "reviewed and approved" until the Billing Dept could call the Online Commerce Department to actually see if I ordered the unlimited plan with the phone.