If you wish to stay with AT&T, here's a few suggestions...
Not that you need to, but I too just resighned with AT&T with the Treo 750. It runs Windows Mobile 5, with (hopefully) an ugrade to WM6. Got 3G connection, and it's REALLY fast...at least within the SF Bay Area. It's not too clunky, got the physical QWERTY keyboard, calendar...all that stuff. Only thing that I would watch out for is battery life. The OEM battery would last all day, but thats about it. If you so think about this...I would recommend an extended battery...but that would add just little more bulk to the phone...but not too bad. I'm loving my Treo...:)
The other that I would think of is the Pantech Duo. It's a small compact candy bar phone that would slide out one way to expose the number pad....slide it another way would open up the QWERTY keyboard. It's also 3G enabled, but since that I dont have it, I can't say anything beyond that. It runs WM6 though.
Then you have those made by HTC. The AT&T 8525 and the newer AT&T Tilt. Both are true PDA SmartPhones, but the Tilt has the advantage of having a 'tilt' screen for better viewing and comes with WM6, as the 8525 still comes with WM5...but can upgrade through HTC's web site. Both can get the job done, even with 3G, but you really do sacrafice in bulkiness.
If you like to unlock...there is the T-Mobile Wing. It's basically the same as the Cingular 8125, meaning that it has EDGE connection, instead of 3G, but has WM6. Also it is alittle slimmer than either 8525 or Tilt with a non-slip feel to the body.
Well, that's my two cents on what is out there. Good luck.
PS-as what you may find out reviews about the Treo 750, I don't experience those negatives on mine.