I think that until recently, the Android market and the iPhone markets had only a little overlap; there were relatively few people that had a perfectly equal inclination to purchase an iPhone or an Android device, assuming that both were available on their carrier-of-choice.
Meanwhile, AT&T has continued to be one of the top complaints about the iPhone. [1] (This doesn't help Apple's image, BTW; if Apple is so concerned with the end-user experience, then why are they in bed with one of the most-hated companies in the U.S.?)
In other words, Android has been a popular fall-back for people that wanted a solid smart phone without AT&T. Although Android has been doing very well for Verizon [2], I have no trouble imagining that Verizon has lost out on a sizable amount of business from the market segment that preferred the iPhone to Android. Android 2.2 came out just a couple of weeks ago; back in July, a Verizon iPhone could have been a big deal.
Now that Android has finally caught up to the iPhone (IMO), I think the holdouts are going to start picking up Android devices, which is going to cost Apple, which has already fallen behind Android in terms of both growth and popularity.
On the other hand, AT&T has been paying Apple by the metric ton for the exclusivity deal, so maybe that money has been worth the lost market share for Apple and they just don't give a shit.
[1]: http://dvice.com/archives/2010/06/top-10-complain.php
[2]: http://gigaom.com/2010/10/05/nielsen-android-surges-to-no-1-...
edit: I suspect that a lot of this comes down to some politics and personalities. Apple had to have pissed off Verizon a bit when they offered a hugely popular device, but only for Verizon's only real U.S. competitor; then Apple pissed off Google shortly after; then, not too much further along, we start to hear that Verizon and Google are getting real friendly with each-other, possibly in an "any enemy of my enemy is a friend of mine" way. Verizon's been happy to push Android real hard, Google's been happy to support Verizon, and the two of them together are certainly putting some hurt on Apple/AT&T. It just sucks that consumers are getting caught in the middle.