As I said above, MAKE SURE you cover your bases. This week (August 2012) I wanted to use my bipap machine on a flight from LAX to Auckland. And was told in no uncertain terms that my machine was not allowed to be plugged in to the medical outlet on the plane. This, despite my previously having been given permission to do just that. After about an hour of being made to feel like a sneaky little liar and trying to get them to let me plug in, the Captain finally gave his permission. It clearly states on this AirNZ website that my machine is approved for use on the plane I was on, and for use in the medical power outlet. So again, MAKE SURE you double and triple check so you don't end up in a tail spin.
NOTE: You NEED to have a lead with a USA plug for your bipap. Not a conversion plug, but a lead with the plug attached. Apparently the conversion plugs can cause arcing so they won't let you use them. I have also found that it depends on who you ask as to whether or not you can fly with a conversion plug, or the USA lead. As I said, make sure before you fly!