[Satrina]

To answer all the speculation:

The Technical
This is based off of the current WLK beta build. Things could change between now and release

There are two threat functions, tied to events that fire during combat (UNIT_THREAT_LIST_UPDATE, UNIT_THREAT_SITUATION_UPDATE)

First, they define two terms:
raw percentage The percentage of the threat when divided by he threat of the mob's current primary target. Using raw percentage You will pull aggro when your threat percentage is 110%/130%

scaled percentage Your theat scaled such that 100% means you will pull aggro, and thus this % cannot be higher than 100% under normal circumstances. (That is, this one has been divided by 110% or 130% as appropriate.)

Now, two functions:

state = UnitThreatSituation(unit, mob)
Returns the unit's "threat situation" with respect to the given mob.

state can be one of the following values:
nil = you aren't on the mob's hatelist
0 = you are not tanking and are under 100% raw threat (good to go)
1 = you are in the 100-110% or 100-130% range (danger)
2 = you are currently the aggro target, hopefully the tank, but someone is catching up and has passed 100% of your threat
3 = you are currently the aggro target, and everyone else is under 100% threat

isTanking, state, scaledPercent, rawPercent, threatValue = UnitDetailedThreatSituation(unit, mob)
Returns several values with detailed information about your threat for the mob:

isTanking is true if the unit the primary target of the mob

state is the unit's threat situation, as above.

scaledPercent is the current percent threat of the unit, scaled in the 0-100% range based on distance from target.

rawPercent is the current percent threat of the unit relative to the primary target of the mob.

threatValue is the amount of threat that the unit has on the mob's threat list. This is roughly approximate to the amount of damage and healing the unit has done.


How that plays out in game
As far as I can tell, the default UI implements only a simple graphic threat indicator on the unit frames, based on your threat situation (the first function above):
0 = No indicator (why not a green indicator, I dunno)
1 = Yellow indicator
2 = Orange indicator
3 = Red indicator

That's pretty simple and will do for many people.

Some people will want more, of course, so they have provided the detailed function which will allow Omen to keep going and be much simplified than it is currently, and also make it a lot easier for alternative addons to be created.

The biggest simplification is the threatValue return from the UnitDetailedThreatSituation. That alone will remove a huge amount of code from Omen, since it no longer needs to calculate itself. Omen really just becomes a fancier display wrapper for these functions than the simple one Blizzard provides in the default UI, which is in line with the base UI design policy.

I'm sure we'll see a number of Omen alternatives released with varying degrees of complexity. I'll probably be looking for something that gives a top-10 name/percentage list and that will be that.

Edit: Some have commented on a screenshot that's going around showing an apparent threat indicator in floating combat text. I can find nothing like that in the code for Blizzard's default UI combat text, so I can only assume it is from a beta version of SCT/Parrot/Mik's

[Wrath]

hmn thx for that nice overview Satrina

Sounds very promising and should be far more accurate threat calculation then these days...

[Kazeyonoma]

awesome, the less I have to dl and update or run in the bg to use up ram, the better.

[Satrina]

Unless you have less than 1GB of RAM in your computer, you're completely wasting time worrying about how much memory your addons are using.

[Kamani]

I think I remember reading somewhere that currently Blizzard has only added the barebones of their threat meter. It was just a quick implement. I believe that they will have their own UI to show threat similar to Omen currently.

[Stubbi]

It would be real nice to have a /target extension that targetted the lowest threat enemy for some tanking love slaps. As it sits today, we either have to mouse click (sucks) or tab a variable number of times till we hopefully get the right enemy target (not as sucky but still it could be vastly improved by this).

As there are macro extensions already available to set the distance max for tab target selection, this would fit in well.

Dps could use the reverse version with a /targetsafeenemy etc.

[Satrina]

There's a pretty big difference between tab target distance and something that automatically will choose a target for you. Blizzard protected all the targetting functions that made Decursive work the way it originally did (and other addons that chose targets for you) for WoW 2.0, and have stated very clearly that targetting will always be something the player must consciously do. Don't hold your breath.

[Lore]

Regarding the floating combat text thing, that's definitely Blizzard stock UI - I have 0 addons installed and get it. Basically it just pops up when the yellow/orange/red icons change with a message like "Gaining Aggro" or "Losing Aggro".

[obtuse]

The old omen had an AOE mode which I grew to like quite a bit once I discovered it a few weeks ago. It offers almost all the features I would want for multi target tracking while tanking. The only thing that held it back was being display only. I'm researching to be able to put together my own multi tanking addon since right now the omen 3.0 is no longer offering AOE mode.

Basically I want targets I have engaged in combat to show up as bars in a moveable frame. On the bars I'll be able to read my percentage of threat over the next closest person in combat with the target. The target bars will display the raid mark if there is one, and allow me to target that enemy by clicking the bar.

I'm also thinking it could feature its own tabbing command which can be bound, which will cycle targets based on your preference by specifying what order you want the raid icons to be cycled in the mod's preferences frame. If I can find a way I would also incorporate the option to bind keys that will target the specific mark, a bind for each icon, if this is not a restricted function. I believe in the mod I could provide the functionality such that it would only work with targets with which you were already engaged in combat.

I'm certain from there the frame could also be configured to provide "target of" column as an optional feature.

I coded a lot of unrealscript mods and worked with other scripting and object oriented languages but I have not gone into much research yet with LUA.
I have been searching but it does not seem such a targets+threat+marks tracking mod is being worked on/available, or ever has been. Therefore I think I ought to dive in and give it a try