appears that Threat-2.0 has "officially" (by its author) been deprecated. the version listed on Curse for WoW 3.0.2 is a stub (no LUA).
the implication (i suppose) is that LibDogTag-Unit-3.0 needs to be converted to use the Blizzard native threat API.
i took a quick look at the LUA, and given my ignorance of the structure of LibDogTag-3.0, it appeared this would be non-trivial (tracking threat via unitid rather than unitguid, which would imply a roster tracking requirement, plus registering with the new Blizzard events, plus the conversion to the Blizzard threat API, all done in the "DogTag way).
sadly, i have my own addon development fish to fry (i took over the Aloft nameplate addon, now over at WoWInterface), otherwise i would offer to help. i expect you are on top of this anyway.
i am a DogTag end-user (i use CowTip, IceHUD, PitBull, and etc, in my own play). so, i eagerly await a new version of DogTag.
if there is any sort of ETA, let us know. as well, if there is a forum where these things have been discussed, let us know (i searched on WoWAce and did not see anything).