I can tell you for a fact that T6 gear will be replaced by blues around 74ish..ill look for some links. maybe not all..and depends on if you can get yourself some blues. But ( set bonuses aside) most level 74-75ish blue is better or on par with t6 in wotlk.

http://thottbot.com/wotlk/i44443>>http://thottbot.com/i31020 as an example. I know the T6 has bonuses..but i would wear the blues over it.