I've asked this question before and never got an answer (I don't think). I see warnings from time to time about not hanging a bike with hydraulic disc brakes upside down in a garage or on a roof rack because the fluid might 'resettle' in a place where the brakes won't work well or at all, or an air bubble floats to a bad place in the caliber or something.
I frequently hang my bike upside down and have never had a problem. I don't personally know anyone who has had a problem with it and neither does the wrench at the LBS I asked the question to. And wouldn't a rider know as soon as they rode a few feet on their bike that there was a problem (like the brake isn't working)? Or, is it something else that causes a problem that shows up half way down that nasty singletrack?
If I'm running a risk I'll figure out something other than hanging my bike from the ceiling upside down. Any thoughts or experience with this? Is it an actual concern? (I'm running Hayes HFX Nines, BTW).