When making this decision, lots of things have to be considered. MT is out no matter what, as not only do many things in game use it as an abbreviation (main tank, mistell, Montana -- ok maybe that last one is a stretch), but it also leads us to confusion with the other "MT" instance in the game: Mana Tombs. For that reason, MaT is also out, though MagT has found a lot of foothold with many players, as has MgT. I originally thought MT would work well (since Mana Tombs is actually "Mana-Tombs," and would therefore be abbreviated M-T), but I haven't see that too much so far.
But almost surprisingly, the thing I've seen most players using is something that I originally mentioned way back as just a joke. Yes, most players I've seen are actually describing the instance as MrT. Not that that makes any more sense, but it does kind of provide a nice homage to a Mohawked WoW hero.
So in our (un)official capacity as WoW Insider, one of the 50 most influential blogs on the net, we're declaring the winner: from now on, the new Sunwell 5man will be known as "MrT." And we pity the fool who doesn't agree.
Haven't been to MrT yet (or MagT, or MgT, or whatever you decide to call it)? We've got a Kalecgos-sized roundup of info on the brand new five-man, including a step-by-step walkthrough of what's in there and how to kill it.