I was in the same boat myself but through alot of forum searching, I found a version for ATT branded phones 2.25 and an unbranded version 2.27. I went for the unbranded one initially as I'm using an unlocked Cingular branded phone on T-Mobile, but it wouldn't work. Kept saying it wasn't supported for some reason. So I went ahead and tried the ATT version since it was an ATT/Cingular phone originally anyway. That did the trick and I'm officially running 6.0 as of today. Here's the link to the ATT version, hopefully that'll work for you.
http://mytreo.net/downloads/treo-750...cial,1189.html
On a side note, what's the deal with 6.1? Was it officially released for the Treo 750? If so, does anyone have a link or a copy?