Yes, there are new rumblings that Blizzard should aim to renovate old content rather than adding more new stuff onto the end of the game (which, as Neth confirms, is what they're planning on doing right now).
But the question I have for you all is: do we need more new midlevel content? I was definitely on the side of those asking for new midlevel content before the new Dustwallow Marsh, and yet I've only found myself questing there a few times -- I've been doing exactly what I didn't plan on doing, and pushing onwards towards 70 rather than sticking around the midlevels. There's no question that patch 2.3 was a popular patch (it also included those leveling changes, and those were such a huge hit that Blizzard's working on other ways to make the leveling grind easier), but was the new Dustwallow Marsh really all that popular? Did it compare, for "instance," to all the people who played Magisters' Terrace when it first arrived, or all the people who worked on the Sunwell Dailies?
To me, that provides pretty clear evidence that no matter how much we wanted new midlevel content before patch 2.3, Blizzard has more incentive to provide new late game content (casual as it may be). Even if there's lots of calls for new midlevel content and revamping of old areas (I'm still convinced that we need Heroic versions of classic instances like Deadmines and Wailing Caverns), Dustwallow Marsh's revamp seems to show that players are much more interested in having new things to do at the end of the game.