I'm not on AT&T network, not even in the US.

The frequency makes a difference, I had the problem with both 900 Mhz and 2100 Mhz 3G. The person makes a difference, your body, your hand, left vs. right, how you hold the phone, etc. The network quality didn't make a major difference (e.g. 5+ Mbit/s full bars 2100 Mhz 3G and ~2 Mbit/s half bars 900 Mhz 3G, both go down to GSM if I hold the phone for a while).

If I did a phone call, I started with a high quality 3G. When I looked at the phone after a 10 minute call, I was down to GSM. The person at the other end complained that my voice sounds like I have a bad reception, ie. the voice is being compressed heavily.

I'm not blaming the network, never had such a problems, not even with the 3GS. I don't remember when I had a dropped call the last time. I just consider myself the worst possible user for the iPhone 4 antenna design, which I find flawed (like most antenna engineers do), and can perfectly understand that the phone works for some other people.