Yes - the service will 'come and go' just like a regular AT&T 'phone, the iPad and your 'phone use the same GSM technology and both link to those cell towers in exactly the same way. So, when your phone doesn't work, neither will your iPad 3G. The I-95 is pretty well covered though, so you shouldn't find too many problems as long as you don't want continuous coverage. You'll find that most of the Interstate rest areas will have coverage as will, of course, the urban areas en route.
AT&T have produced an informative leaflet explaining all their data plans for the iPad and you can download it from here....
http://www.att.com/Common/about_us/files/pdf/emerging_devices/iPad_fact_sheet_081210.pdf
As Marie explained, you can take the service just for a limited time and suspend it at any time you want on some of their plans - it's a sort of 'pay as you go' service. Anyway, have a look through the leaflet and, if you have any questions, don't hesitate to come back to us.
I hope you have a great trip and please post your experiences on the Forum
Tim