^^ This is the best answer in here.
You can't predict signal with AT&T.
I don't believe the update has anything to do with it.... It's probably more of an effect from the almost 1 million new AT&T subscribers....
More like 400k new subscribers but yea, that many new subscribers over a weekend is more than almost any carrier could deal with. On one hand they are doing pretty well in that their network hasn't completely choked. Just as long as they adjust and quality of service increases in the next week or two.
This also seems to be a good reason why the iPhone needs to be a multiple carrier device.