I still think, though, that this is more of an experimental release on Blizzard's part, rather than a full rethinking of the way dungeons should be done. The Coliseum isn't an instance like we've ever seen before (though Vault is probably the closest) -- it's supposed to be extremely modular, and it's very much a patch-specific release rather than standard content like Naxx or Ulduar. We'll have to see exactly how it works, but my guess is that Blizzard is testing the waters with this and the other raid content in Wrath. Chances are that if for some reason this doesn't work out (will we eventually see raiders running all four modes every week, and finding the content much more repetitive than Blizzard planned?), the idea of simply having hard modes on bosses rather than lockouts probably isn't completely abandoned yet.
Patch 3.2 will bring about a new 5, 10, and 25 man instance to WoW, and usher in a new 40-man battleground called the Isle of Conquest. WoW.com will have you covered every step of the way, from extensive PTR coverage through the official live release. Check out WoW.com's Guide to Patch 3.2 for all the latest!