We've had the Wifi iPad since the first day so we're past the 14 day return policy. Last night we were at the Apple Store and were told by an employee that early adopters are going to be allowed to exchange their wifi iPads for the 3G version and pay the difference.
I find it hard to believe since we'll be past the return period. Calls to different stores were mixed, between it's not true and they'll do it with 10% restocking fee.
I really doubt that they will do this. There is no advantage to Apple to take back a used iPad in exchange for a 3G. Plus anyone who returned it already and ate the 10% would be none too happy...
The only way that this could work is if they somehow got a kickback from AT&T. But since there is no contract (and therefore no guarantee that AT&T would subsidize the iPad) I'm not sure how this would work.