This isn't the first time we've heard about putting a built-in threat meter in the standard World of Warcraft interface. The idea came up back at Blizzcon 07, and Neth confirmed plans to include such a meter in February '08. But there's not been much discussion about the idea since then. Today, Bornakk confirmed they're still working on it, but there's obviously no details to be shared yet. ("Soon," and all that.) So, while we don't know that we'll be seeing the threat meter with the release of Wrath of the Lich King, at least we know it's probably still coming eventually.
Threat meters are almost entirely a mandatory requirement for most raids. The two big ones are KTM and Omen, of course. It lets your DPS know when to let off the gas a bit, before they pull aggro off the tank. Or, maybe, tells the tanks they need to push the pedal harder and build up more of their own threat instead of limiting the DPS. I can't imagine raiding without one, and if that's the state of things for enough players, I expect Blizzard will take it over themselves soon enough.