The Verizon iPhone 4 goes on sale tomorrow and AT&T is not taking it sitting down. Yesterday, the carrier AT&T sent out an e-mail to customers promoting its own iPhone 4 service’s ability to multitask, with the tag line, “only AT&T’s network lets you talk and surf the Web at the same time on your iPhone.”

CNET explains why AT&T’s ad rhetoric has a bit of truth to it: “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.”

Verizon launched its own ad campaign promoting its iPhone service. The ad features people staring at their watches and clocks counting down. A voiceover says, “To our millions of customers who never stopped believing this day would come, thank you.”