OK i found this just then. In storm peaks there is cave call Gram's bane. Its east of K3. To fly in the Cave Wall hug the top of the cave as u fly through the enterance if u dont do this you will be dismounted. (this might work on other caves i havent tried) , Then to get under storm peaks fly to the middle cave and go up in that cavern place till u see a machine thing in the wall. like at the hall of stone. do the same thing u did to the enterance of the cave. wall hug the top of the enterance part and ur through. all stuff in that area doent have collision so you can fly though it. Tested today 8:12PM Australian EST. on a purlpe nether ray mount 280%
EDIT: FLy Fast to the back of the cave till u dont dismount, may take a couple of tries.
PROOF flying in cave >>>>
To get under northrend go through here and fly down till all the red stuff goes around,(when u do it you will see waht i mean) Dont go back through the Cave window thing or u will dismount.