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Since we're only raising the level cap to 85 in Cataclysm, one of the goals is to pack as much content and gameplay as possible into the new 78-85 zones and dungeons. We don't want the leveling experience from 80-85 to feel as if it's only half of the content we've provided in the last two expansions. So, with that comes more substantial scaling of items and character power with each level gained. A level-82 character should feel significantly more powerful than a level-80 character, for example. There are some epic and legendary items from Wrath of the Lich King raid content which may take a while to replace, but chances are most of your gear will be replaced by the time you start running level-85 normal 5-player dungeons.
It wouldn't surprise me if some people are raiding the first tier of content while still using ICC epics and not replacing all of it until the second tier of raiding. It just comes down to how Blizzard tunes stuff. If they want for you to replace every last bit of iLevel 277 gear when you first step into an 85 heroic, they could do that, too.