If Steve Jobs didn't have his fingers in it somewhere, I'll eat my hat.
I can see iJoe Consumer trying to decide between the iPad WiFi or 3G model. Included in the sales pitch is that AT&T's data plan has been slashed in half and only costs $15 now. Now iJoe Consumer pays $15 for his iPad, and depending on what he does with his current iPhone probably opts for the $15 plan on his iPhone as well. Thinking all the while he's getting a great deal because now it's $30 for two devices. Not realizing that that $30 doesn't even get you 500MB of data. Upon realizing it, iJoe decides to opt for the $25 dataplan for his phone, and is now spending $40 a month whereas before he was only spending $30 (and probably gets popped for another $10 a month on top of that when his iPad exceeds his data limit).
If AT&T can entice a customer to purchase 2 data plans, whichever way you slice it, AT&T gets more money. The question remains though... are there enough iPad 3G owners to make it worthwhile?