Aloha Bencho!
I just "upgraded" (depending on who you speak with) from a n95 to a g810. I have T-Mobile but came across some of the information pertaining to AT&T. Note - I don't have AT&T, so that portion is from what I've gathered in the course of researching the phones.
charlyee covered it. For myself, 1900 (T-Mobile) works well. I was curious and tried half-heartedly (for 30 min or so) to find the ATT coverage map by frequency, but came up empty handed. Never saw a single review of the phone on their network either. In general, my phone gets better coverage in Honolulu, Hawai'i than the n95 or Wing that I had.
I did see on some review or forum that U.S. version would be released in the third quarter, perhaps that's your best bet. It's supposed to be a quad-band.
Re: Slider phones: I never took a look at the Sony. My T-Mobile Wing (not sure what model HTC it is) was a good slider phone, albeit hindered by WM6 and a slow processor. I use my phone primarily for work/email, which neither the n95 or g810 (and I'd suspect similar sentiments on other Symbian OS) is good at, in my opinion.
If the slider/cameraphone is the top priority, I went w/the g810. I've owned both Nokia's and Samsungs previously and don't consider myself biased. The g810 had two huge factors which I disliked about the n95 - 1) picture size (can search around on google, widely documented. Something with Nokia's compression settings), which I didn't see on any reviews before I bought the phone and 2) n95-3 doesn't have a camera cover. However, I feel indoor and night pics are better on the n95.
Battery life is about the same for both phones - sub-par. I can get through the day w/minimal use w/another quarter charge throughout the day (typical: receiving 40-60 emails, a few texts, 30 minutes of calls, a couple of pictures).
I'll type up a review someday.