So remembering back when I was raiding Hyjal in TBC, those annoying invisible walls prevented me from exploring some. And since the uninstanced hyjal is a bitch to get to these days, It made me go booboo. So I decided to venture back to find a way to get past the borders, and sure, after 2-3 hours of jumping around I found a way.
Quick video showing where and how to get there: [ame=http://www.youtube.com/watch?v=RvyWq0zNDso]YouTube - Hyjal summit exploration[/ame]
(Just uploaded it, didnt bother rendering in good quality but it should be visible. a bit bad quality until its properly processed.)
Starts at the horde camp. didnt show more than this, do it yourself and see what you find. :-)
Oh, and once you jump down where I did in the end, you are past the walls. However you can't really get out since the invisible walls will prevent you from going, have fun running into that :P (Also the onyxia cave thing isnt there for some reason. pff)
Ehehehnjoy.