That sounds high to me, and indeed, Johnstone admits that, as time goes on, that number will probably be much lower. Not only are AT&T's plans tough to get out of (and therefore not really worth the trouble to switch), but even if you do, they've just upped their termination fees, which will put another roadblock in the way. And Johnstone also says that AT&T's coverage is getting better by the day, so by the time Verizon does have the iPhone, AT&T's technical problems might not be so bad.
Even if the number isn't quite as high as 40%, iPhone exclusivity has been a huge boon for AT&T in the past, and losing it won't be good for the company. If Apple announces a Verizon deal later this year, investors will be keeping a huge watch on what AT&T ends up dealing with.