iPhone 4 Teardown
In 3GPP cellular networks, the decision to move from one frequency to another is up to the network to decide. The terminal can only say "I would like to go to this frequency or neighboring cell". Further it is not likely that the phone is measuring congestion of the network. Once again that is a parameter that the network should be monitoring and should be moving the terminals around (also called a handover) from cell to cell and frequency to frequency.
In reality, the AT&T network gear vendors have probably improved their handling of congestion.