Macblack,
Yes I have travelled with my iMac on a plane. Of course you have to check it in as cargo---not cabin baggage. No airline will let you hand carry somehting that big. And you will have problems with airport security, for sure.
First I tried to hand carry mine as cabin baggage with the iLugger Bag ( see this link:
http://luggerbags.com/) to see if they would let me in---they did not, citing security and that it was way above the allowable dimensions of what qualifies as hand-carry luggage, and instructed me to check it in cargo baggage. I of course thought better about this because, while the iLugger is a convenient way to carry the machine around, I wouldn'ty trust it unaccompanied in the cargo bay (I don't feel confident the pads are that thick for this particular purpose).
Suffice to say, I promply rescheduled my flight and went home to pack the machine in the original box it came with---with the stryrofoam padding and everything. I felt that if Apple uses this packaging to ship their machines, then it should hold up in an airline cargo bay for the short (4 hours) flight.
I checked that in, arrived at my destination, and the iMac held up very well---not a nick or scratch in it.
Hope this helps you out...