Why would Apple lock itself into another exclusivity deal with the carriers this time around?
I'd expect it to use WiFi and have the ability to work with the carrier of your choice by adding a SIM card if you want but not required. Especially if this device is destined to replace the low end MacBook. Not everyone needs internet everywhere. Especially if it is a larger device. For school work or home, you probably have WiFi access already. But cell carrier connection would be available for those who want it, just not a requirement at purchase.
Do you think they might do something like Amazon's Kindle where it uses cell technology but the user doesn't have to worry about contracts or bills?