To at least get everyone on the same page in terms of information, since a summary of a translated version of a translation of a few sentences in a single interview may not be the best foundation for a 24-page discussion, here's an overview of our current thinking:
We will have Normal and Heroic versions of our max-level dungeons. (Note that we did not have Normal level 90 dungeons in Mists.)
There will be no special requirement other than basic level/ilvl requirements on queuing for Normal dungeons or LFR.
Level-up quest gear will get you into Normal dungeons/scenarios, and Normal dungeon/scenario gear will get you into LFR.
Heroic dungeons will be more challenging than the Normal version (not brutally difficult, mind you -- just somewhat more demanding).
You will need a Silver Proving Grounds medal in a given role in order to queue for random matchmaking for a Heroic Warlords dungeon. If you form a premade group, you can zone in regardless and no such requirement applies.
In the past, challenging content and random matchmaking have often not gone so well together. Relying on item level does nothing to prevent a random group from getting, for example, a tank who may literally have never tanked before. Everyone has to learn somewhere, but we'd rather not have that learning come at other players' expense. One option would be to require a full premade group to do Heroic Dungeons, as we do with Heroic Scenarios, but that would present a very high barrier to entry (finding 4 other people with rigid role requirements is much harder than finding 2 other people with no role requirements). This solution represents a compromise in order to allow the content to see a broader reach via matchmaking, while minimizing frustration.
And yes, we realize that by formally using Proving Grounds as a qualification, it will be incumbent upon us to further refine their balance and mechanics. We'll be updating them for Warlords, and we're confident that we can make them a fair test of baseline ability within a given role.