I am also with Cingular/ATT and have used my phone (not an iphone) on my last two trips (Italy, China - mainland and Hong Kong). Before I left, I called ATT and had the international roaming plan activited. The small monthly charge for the plan lowered the per minute charges and maybe also the data package charges as well. (don't remember) The signals were fantastic. I used it sparingly, but it made quick calls home much easier than any other method I had tried. I was in a situation in both cases, where I needed to be able to be reached without a problem. I used the email feature on the phone in China and it worked very well.
I also took a phone with an Italian SIM card with me on the Italy trip so that I would have a local number. I had students with me and needed them to be able to reach me easily. It worked well for that purpose and for making local calls. The phone and SIM card were purchased through telestial.com - chosen, after much research, as the best for our needs