When you get there you'll want to talk to him, make him pity the fool. And in return you'll get a five Mohawk Grenades that let you make others wear a Night Elf Mohawk mask. You can get more grenades once your five are all used up.
There is a 50 minute buff called "Mohawked" with the flavor text of "You pity the fool" that gets placed on you when you wear the mask. You can right click the debuff to remove it.
Check out the gallery below for images from this little Monday afternoon surprise. And just for kicks, the original Mr. T WoW commercial after the break. When you watch the commercial again, note how Mr. T's in-game self looks exactly like his character does in the commercial, quite neat!
Update: There is now an FAQ on the Night Elf Mohawk, which is reproduced after the break.
From the official FAQ:
Q. What is the Mohawk Grenade?
A. The Mohawk Grenade is Mr. T's latest invention: an in-game item that, when hurled at another character, gives everyone within the blast radius an instant T-riffic haircut. Don't worry, you and your friends will look gooooood.
Q. Where do I get it?
A. Quit your jibber-jabber and claim your Mohawk Grenades from any of the Night Elf Mohawk NPCs camped outside every starting zone.
Q. What regions are eligible to claim the Mohawk Grenade?
A. North America, Europe and Korea.
Q. Is this a permanent item?
A. Each Mohawk Grenade has five charges before exhausting its power, and you're welcome to claim another once the charges are gone.
Q. How long will the Mohawk Grenade be available in-game?
A. Claim Mohawk Grenades starting November 16th in North America and November 24th in Europe and Korea. They'll be available until... well, until the Night Elf Mohawk decides his work is done.
Q. Where can I see the Mr. T commercials?
A. The commercials will be airing on major TV networks throughout North America beginning on November 16th and in Europe starting on November 27th.