Cingular was updating their network to improve the minimum speed (which is what you get if the cell you're using is full of users) from 40kbps to 80kbps.
So it's quite possible that your cells are usually full up, and the upgrade has helped. Either that, or everyone around you shut their PDAs off
They're also trying to update their entire network to handle HSDPA (3G speeds), for their other phones at least. Who knows when/if the iPhone will get upgraded.