I checked and I have unlimited data. I stepped through the upgrade wizard for the 4S and saw this.
This device requires selection of a $30.00, $50.00 or $80.00 per month data service and a messaging option to process this order. If you currently have certain existing data or messaging options (e.g. Customers with $29.99/Unlimited data plan or a bundled plan with data and messaging) you can still place your order at this time, but such features may drop off your plan.
Upon receipt of device & activation you can have your existing options put back on your plan by calling customer service at 1-800-922-0204.
// This seems to be double-talk for, "we are taking it away and hopefully you won't know you had it.