Sony Ericsson Z550a

I'm considering buying this phone as I had good luck with a Sony Ericsson Z520a. Looking on EBay, there are no Z550a's currently for sale, but Z550i's. I assume this is an international version, it seems to have the same specs, but I'm wondering if this will work with AT&T (Cingular). It's unlocked, and AT&T says they will provide me with a sim chip at no cost. Also, are there any other differences between the Z550i and Z550a?