It looks like the Battle of Mount Hyjal has been opened for testing on the PTR to everyone with Honored reputation or greater with the Violet Eye. Boubouille of MMO Champion, Bistustal at World of Raids, and the stalwart testers of the Elitist Jerks forums have gone and taken some lovely screenshots for those of us not on the PTR.
You start out at an Alliance base commanded by Lady Jaina Proudmoore, who is apparently both 20 feet tall and totally okay with the non-scourge undead. The Scourge come over the hill in Black Morass-like waves. Without the help of your team, the Alliance and Jaina get owned, and everything despawns.
According to the lore, after the waves at the Alliance base, you should go to the Horde base (commanded by Thrall) and the final Night Elf base (commanded by Tyrande.) You'll fight your way up the mountain until the final battle against Archimonde the Defiler. Wowwiki also suggests that you fight two other major lore figures from Hyjal -- the dreadlord Anetheron and the pit lord Azgalor.
All in all, Mount Hyjal looks like a fun instance. Too bad I'll probably never see it until level 80. What kind of instances would you like to see in the Caverns of Time? Do you wish you could run Hyjal?