The small sample around me (about 20 users in their 30s-40s, mid-top management types) all move from WIFI to 3G iPads. We're not in US, so I can't comment about the AT&T situation. But from the iPad users I've seen: >90% are happy with their device, brings their iPad on vacation than a note/netbook, and they usually end up upgrading to 3G version if they have the WIFI.

Also... interestingly, only about half of them were Apple users in the past, and after 'experiencing' iPad about half of those took up iPhone 4.

e: er, my point is, once the users move to tablets, they realize that they can do some 60-90% of their mobile work on their tablet and they'd more likely opt to get the always-connected option.