That was the public excuse that Steve Jobs brought up at the AntennaGate conference to help try to explain iPhone 4 reception problems.
But while the timeline for new towers might be valid, it's not really pertinent:
There's no need to add new towers in cities like San Francisco. Third parties already own towers at every prime location, and lease them out.
Yet those locations may well not be optimal for AT&T's network. So AT&T would still have to go through channels to get new towers up... and Steve doesn't normally defend idiocy. AT&T can do better, much better. Right now, New Orleans went from 1.6Mb/s down to 3-4Mb/s down average; if AT&T can do that here, why not elsewhere?