Might not matter how long you've been with the company. I had a 3G for a year. Turned it into insurance through Best Buy for the 3GS a few months ago. Boom, no longer grandfathered in, even though the at&t rep GUARANTEED ME ON THE PHONE I would be. :'(
Called them up, and they said there literally is no where in the system they can choose the option for unlimited. Why they couldn't just adjust my bill every month is beyond me.
For every 1 mg you go over the 2mg, it's an extra $10 a month, right? Something like that. Not that much if you just go over a little bit (within a gig), but a fuck ton if you go over a lot (a few gigs).
Edit: To clarify, our family had a Motorolla flip phone in 1993 through whatever at&t was back then. So we've been with them that long.