Last night, Impervious took a break from their raiding grind on the live realms to check out Hyjal. They had a developer on hand, no doubt to work on fine-tuning the bosses as Impervious attempted them.
They went through a total of four bosses - two at each camp location - so it's safe to assume that there are at least six bosses in the instance -- that is if the format continues at the night elf camp as well.
The bosses in order of appearance were: Rage Winterchill, Anetheron, Kaz'rogal, and Azgalor. Check out the images below - including loot for Rage Winterchill(death touched) and Kaz'rogal(killed) - and be sure to check out the Impervious website for more info on their trip to the PTR.