AT&T just announced this morning (in response to accusations that it throttled upload speeds) that a software update for its Alcatel-Lucent backend caused the glitch and they are working on a solution (no ETA).
I ran speedtest last night on my iPhone 4 and was getting really fast download speeds, but slow uploads.
Here is their statement:
AT&T and Alcatel-Lucent jointly identified a software defect - triggered under certain conditions - that impacted uplink performance for Laptop Connect and smartphone customers using 3G HSUPA-capable wireless devices in markets with Alcatel-Lucent equipment. This impacts less than two percent of our wireless customer base. While Alcatel-Lucent develops the appropriate software fix, we are providing normal 3G uplink speeds and consistent performance for affected customers with HSUPA-capable devices.
Not sure if that could explain your slow download speeds, although I wouldn't be surprised.