To find the past of Grim Batol, you've got to read Day of the Dragon, the novel by Richard A. Knaak. Basically, the Dragonmaw Orc clan had captured Alexstrasza in Grim Batol, and was using her to make their own dragons. Rhonin (who will be the leader of the Kirin Tor in WotLK) was able to break her out and free her. And that didn't make Deathwing (Onyxia and Nefarian's father, and the big bad black dragon aspect of Azeroth) very happy, because he was trying to steal Alex's eggs to make his own dragonflight. Rhonin eventually defeated Deathwing, and there DotD ends.
Cut to now. No player has ever been inside Grim Batol, and the only place it's ever been used in the game is as the spawn point for a dragon in the Horde's Onyxia key quest. Meanwhile Deathwing is MIA, or maybe even kept prisoner in Grim Batol (rumors say the Red Dragonflight may have taken him prisoner in there to get revenge).
However, things (as usual) are not always as they seem. Lately, there have been murmurs (in Outland of all places) that Deathwing is back. Grim Batol may not be a simple fortress-- it may consist of the whole coastline on the eastern side of the Wetlands. And if Deathwing is free, I've heard rumors that he, like Rhonin, may be found up north-- Northrend's Dragonblight is a place where dragons go to die, and Deathwing is supposedly headed up there, claiming that all he wants is to end his life in peace.
Of course, he's not to be believed. And the Red Dragonflight is doing something in Grim Batol, even if it's not keeping Deathwing prisoner. Only time will tell what Blizzard plans to do with the big fortress overlooking the Wetlands.