I know I'm not the first to say it but I think AT&T's service, at least within the past year, has greatly decreased it the southern california area. I use to get full 3G, 2.5 mbps download speeds and 1.5 mbps upload speeds in my house and now I can't get over 1 mbps and I have 1 bar in my house. Dropped calls and increased dramatically and it sucks when I have to wait 20 seconds for a phone call to connect (hear the ringing) or to simply drop altogether. If Verizon gets the iPhone, AT&T will lose a lot of customers. They simply have no leg to stand on. Their future is all going towards 3G towers to help with the backup and let's be honest, their not going to have any LTE worth bragging about until the end of 2012. They can't even brag about their "Fastest mobile broadband network" slogan anymore because they simply aren't.
With all that aside, it boils down to how the service is in your area. If AT&T sucks but you have to have an iPhone, then once it comes out for Verizon, the obvious choice is to switch (assuming the service on Verizon is good in your area). However, if AT&T is great and you haven't had a problem, why would you switch?
I think more people are getting fed up with AT&T and their lackluster service. Before they were #2 or #3 on the customer boards but now they are dead last and for good reason. I really can't wait for Verizon to get the iPhone because it will hurt AT&T (remember the study, upwards of 30% of iPhone users would switch?). I'm not saying everyone is going to switch at once, but Verizon will steal a lot of AT&T users. Remember, "free" market society, competition always benefits the consumer and with the iPhone going to Verizon, it will create one hell of a competition for AT&T to try to hold onto it's already dangling iPhone customers.