We've suspected this all along, but now Bornakk has confirmed that yes, when the next expansion comes out, we'll all be turning in our epics for green gear once more. (And via MMO Champion, here's a comic that explains just what you're feeling right now). As he says, the whole point of a subscription MMO is that you need to keep playing to be awesome, and so new, more powerful gear is exactly where Blizzard wants to go.
On the one hand, this is actually a great thing. I just finally brought my Hunter alt through Hellfire Peninsula, and it was terrific to run a few quests and all of a sudden have what used to be raid level gear. And as Bornakk says, another reset in Wrath of the Lich King will accomplish the same thing for new players then (including Death Knights, ahem, so if you're reading between the lines as much as I am, that means that Death Knights will probably start before level 70, because they too apparently will be coming through the 70-71 gear transition). It is great to do the starter quests in a new expansion, and quickly get brought up to speed with some of the best gear in the game.
However, now that this is known, does it mean the gear you earn now is worthless? I don't think so-- while the old endgame was almost all about gear, there are so many epics now and so many ways to get them that the game is much more about how you play rather than the gear you're playing for. Sure, we'll all end up stashing our epics away for a green quest reward from the Borean Tundra, but we'll always have Karazhan and Gruul's, right?