One difference between CDMA and GSM networks (and, of course, handsets) is that with GSM, people can browse the Web on their phone while talking; with CDMA, they can't. That's just how the technology works. And AT&T wants iPhone customers to know it.
In a new ad that AT&T is pushing to its customers via e-mail, and that GearLive just posted, a woman with an iPhone is seen booking movie tickets while talking with her Bluetooth hands-free kit. A line of text informs customers that "only AT&T's network lets you talk and surf the Web at the same time on your iPhone." Which is true (unless, of course, one's using an unlocked iPhone on T-Mobile--but who does that anymore?).
The e-mails are a reminder from the carrier to its users to keep that point in mind as the Verizon iPhone gets set to appear. Though some users are expected to switch carriers when that happens, the number likely won't be as large as some had thought. That said, AT&T naturally wants the number to be as low as possible, so expect more fun like this between the two carriers in the next couple of weeks.