I've been speculating a lot on what Blizzard is going to do with the old instances, just because I'm always ready for a challenge, and currently, at level 70, the old raids don't offer a full group much of a challenge at all. But as some of our wise commenters have said, you don't have to wait for Blizzard to change the old instances to have fun with them-- just go in solo.
NSDragon on LJ asks what instances (Mara and up) can be soloed at 70. You don't have to wait until 70 to solo Mara-- I know folks who were soloing that one back at 60. Sunken Temple might have been tough at 60, but a well-geared 70 should be able to take that place down.
I know for a fact that a guildie of mine has repeatedly soloed BRD all the way to Emperor for farming, and a hunter in the guild was farming Stratholme for Righteous Orbs a while back. So I'd guess that most of the 5mans can be soloed pretty easily. Has anyone tried to beat UBRS solo? That seems much harder, but a duo seems pretty easy, and I'm sure a trio of the right classes would probably take down Drakk before he even knew what hit him. Someone on the LJ thread says MC has been 5manned, and I've heard that Ony has been three manned. Anyone know about BWL? Or Naxx? It's really hard to believe that Naxx doesn't still require at least 15-20 well-geared 70s, but I guess anything's possible. ZG and AQ20 are probably easy with ten 70s, but I could definitely see those getting 5manned as well as more people head into the new endgame.
When a five man group downs Kel'Thuzad, you know it'll be time for Blizzard to worry about the old instances. But until then, I guess unguilded 70s can just enjoy all the new solo content!