That is the latest most up to date version of 4.2. You can use which ever flavor you decide, but you are not tied to the AT&T version, if you choose you want one with litle more advancement.
I don't know why you think AT&T does not "support" or even have 4.2 for the 8700 on their website, they do, I just looked
Quote:
Software For BlackBerry 8700c (AT&T)
BlackBerry Handheld Software v4.2.1.155 (Multilanguage)
Package Version: 4.2.1.155
Consisting of:
Applications: 4.2.1.96
Software Platform: 2.3.0.79
File name: 8700M_PBr4.2.1_rel155_PL2.3.0.79_A4.2.1.96_ATT.exe
File size: 61.77MB
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Link -->
https://www.blackberry.com/SoftwareD...ient=Rc4cZBaBN