We don't know exactly when we'll be allowed to get in, but we do know HOW we'll get in (most likely): The aforementioned Plates of Uldum. But when you think about where we might find the plates we need, the most obvious answers suggest that we may not be all that far from opening Uldum. Consider this: the best place to find a copy of the plates would probably be in another Titan stronghold, right? Enter Ulduar, a Titan fortress (now apparently occupied by Storm Giants) located in Northrend. We know from Blizzcon that it will be a WoTLK dungeon, so it's definitely coming. It's the perfect place for the Titans to stash a copy of the needed plates. Of course, if that doesn't pan out, Drysc also says he believes that something has broken out of Uldum in his post. Perhaps whatever it is (A Golem? A Giant like Ironaya?) bought a copy of the plates out with it, and if we track it down, we can have them for ourselves. Either way, WoTLK seems like it will offer plenty of prime opportunities for the gates of Uldum to open, if not with the expansion release, perhaps later as a patched-in dungeon like Dire Maul or Magister's Terrace.
Of course, the other question would be... what would we find in there? We're not really sure. Right now, there's quite a few giants milling around outside the instance. We don't know yet why they're attracted there, or if they've managed to get into Uldum itself. Perhaps they're attracted to some piece of Titan technology inside the fortress, or perhaps they were set there to guard it by the Titans themselves. We might also expect the Earthen and their "Unacceptable variant," the Troggs, to make an appearance again. We might also find more strange creations of the Titans. We don't quite know yet what the Titans might have up their sleeves, or what they might have stored away in their fortresses.
As far as what knowledge and lore we could find inside, the possibilities really are endless. The Titans apparently created much of what walks Azeroth, so the origins of many species and primeval history could be at our fingertips. We could find out more about the Old Gods they fought, or the Dragonflights they established to watch over Azeroth. But even if you just stick to elaborations on the Discs of Norgannon, the possibilities are many. We could find out more about the exact origin of the Dwarves and how they - and the Troggs - evolved from the Earthen. Branching off from this, we could also finally find a point of origin for the Gnomes (I have often wondered if their affinity for creation was implanted in them by the Titans), Humans (There's been some speculation, backed up by the Warcraft RPG books, that they could also be a permutation of the Earthen), and Goblins.
It really does seem to that of all the upcoming content, dungeons like Ulduar and Uldum could be the biggest source of new old lore to come out in a long time, and could answer a lot of old questions. Hopefully Drysc's post is a sign that those floodgates will break sooner rather than later. I know I'm eager to dig into it all.