Actually, in Halls of Stone you get a very similar disk from the Tribunal of the Ages, and you even talk to the same NPC (the *something* of Norgannon, I seem to remember), so yes, the story is definitely continued but in Ulduar, not Uldum.

Also, Uldum is confirmed to be a whole new zone in Cata, with 2 instances :)

Uldum is (was) a unused gate in Tanaris. It's still there in the south of the zone, so check it out before Cataclysm comes ;) It's a broken gate surrounded by mountain giant guards.

In Cataclysm, we will go into Uldum (through the gate I presume) and discover that there is a whole zone behind it which was somehow 'invisible' to us before (I think of it as a Doctor Who-like 'perception filter'). It will apparently be an Egypt-themed zone, with pyramids and cat-like stone people (I guess similar to the anubisath).

The rumor among the factions (Horde, Alliance, Twilights, possibly more) will be that the Titans were working on some kind of superweapon at Uldum (perhaps the Algalon failsafe device?), which is the main incentive for us to go in there.

And yes, the Lorekeeper of Norgannon that you encounter in Uldamar also shows up in WotLK (well, maybe not the same individual one.. they are some kind of librarian construct).