The clearest motive, as my colleague JR Raphael points out, is AT&T's need to join the Android party, as all three other major carriers have already shacked up with Google's operating system, or at least plan to. With one analyst from Gartner expecting Android to overtake the iPhone in global market share in 2012, AT&T could be planning ahead.
Moreover, AT&T needs to prepare for the day when the iPhone is no longer exclusive. Even AT&T says that's inevitable, so the carrier will want another exclusive touch screen phone to fill the void. Sure, the carrier has a nice line-up of Blackberry and Windows Mobile phones, but those lend themselves best to business uses, while the iPhone does not.
It's possible that the Dell smartphone could even look like the iPhone. Depending on who you believe, AT&T's Dell phone will either be similar to the Mini 3i Dell is introducing in China (that's the Journal's take) or basically the same phone with a few enhancements (says CrunchGear). While AT&T won't want an iPhone clone, some similarities could be helpful, as customers would see Dell's phone as a clear alternative to the iPhone.
If I'm right about all this, then there's just one snag: AT&T would be putting its faith in a computer maker that, before the Mini 3i, had no experience with smartphones. Dell is an unproven manufacturer, so this rumored deal is riskier than going with, say, HTC.
Then again, rolling the dice on a computer company with no smartphone experience worked out swimmingly for AT&T last time around.