In spite of phenomenal growth rate in the AT&T subscriber base, i.e. 2.6 million subscribers added in Q3, 2010, it is predicted that iPhone- AT&T users will shift its base to Verizon from AT&T. Verizon cannot compete with AT&T in terms of its subscriber base.It is clear now as to why Verizon has become too friendly with Apple despite tons of its android model.
The iPhone service on AT&T has not improved. Dropped calls are a major issue with AT&T. The reason for its dropped calls is not due to lack of bandwidth but related to signaling' control and status information that need to be communicated back and forth across wireless networks. Also AT&T subscribers will be to rely on AT&T's 3G network for another 18 months or to switch over to Verizon. Verizon smartphone subscribers use more data compared to iPhone customers on AT&T and hence it is believed that the Verizon network will support iPhone. Android phones also use power saving disconnect methods that iPhone popularized.
The interesting news is that apple is going to supply dual mode GSM/CDMA iPhone in Mid 2011, which would support carriers worldwide. The customers going in to Apple shops to purchase iPhone are likely likely to go in for Verizon. The delay in launch of 4G coupled with Apple's plan to support GSM/CDMA is definitely going to hurt AT&T. Anyways in the whole game, Apple is the clear winner.