In reply to ebdon:
Same here. I just use my Vaude Croz 48 (great rucksack, sadly no longer made) with the lid and hipbelt off and straps pulled tight. Never had a problem flying with it.
I find rucksacks that are specifically designed to meet cabin luggage limits, like the DMM Flight 45, just that little bit too small to use as a general purpose cragging bag. (And no ice axe loops)