If you clicked on the link and watched the Apple Ads you saw that their response to the Verizon ads was to highlight AT&T's 3G network's ability to support simultaneous voice and data.
Verizon started this whole thing by hitting AT&T and their customers where it hurt: AT&T's coverage. This has been a major pain point for iPhone users around the country as many users in major cities don't get to experience the advantages of AT&T's 3G network due to the congestion kicking them down to Edge coverage (which doesn't support simultaneous voice and data).
In my experience there aren't a lot of Verizon users complaining about their inability to access data while on a phone call. However, there are a lot of iPhone users complaining about their inability to access AT&T's network at all.
This is a problem for Apple and AT&T because potential customers are buying into Verizon's ads. They see these ads and it resonates with all the complaints that they've heard about AT&T's service. So far the iPhone is still the best smartphone you can buy on any network and it's not as if Apple is having a hard time selling them. However, both Apple and AT&T need to step up their response to Verizon's attacks to show that they still have the best, most revolutionary smartphone on the market.
