The AT&T firmware is in the download area - in typical US cellular carrier fashion, all parts of it are carrier specific:
W760_R3CC005_MAIN_ATT_DE_RED52 <- Main firmware
W760_R3CC005_FS_CINGULAR_RED52 <- File system
W760a_1212-6828_R10A_ATT_USA <- Custpack
Also note that the AT&T/Cinguar phone is designated "W760a" - but I guess you already know this :)