Looks like T-Mobile is charging people a Pay Per Use Data Plan for anyone using a Smartphone. Slickdeals has a long thread about this problem. Apparently T-Mobile notified people by Text Message about this change.
T-Mobile’s Official Statement follows:
T-Mobile customers without monthly data plans now have access to data on their handsets via Mobile Web Pay Per Use. For $1.99 per MB, T-Mobile customers have the web at their fingertips and can add a different data plan to their service plan as their needs may grow.
Before the Mobile Web Pay Per Use feature was added to their handsets, T-Mobile customers were notified via bill insert, SMS, email, or at the time they signed up for or renewed service. In addition, prior to using the web on their devices for the first time, customers should receive an alert notifying them of the $1.99 per MB charges. T-Mobile customers may opt out of this data feature by calling T-Mobile Customer Care at 1-800-937-8997 or by visiting their local retail location.