This past summer I built a comp for my 10 year old who lives near Edmonton Canada, I live near Seattle. I was highly concerned about mailing it using any method because of the potential for damage that just one good drop could cause, especially in light of the Zalman CPU cooler I used and the added stress to the MB. I considered driving it, but 17 hours one way would end up being a hell of a road trip.
So, I went to the local Walmart store and bought one of those foldable metal luggage carries with wheels on it. I put the completed comp back in the box the case came in, and wheeled it right up to security with my Ticket in hand to fly it there myself. The only requirement they had was that it was able to go in the xray machine. This was a Horizon Air flight and you can check baggage right at the nose of the airplane before you load on what they call Ala cart. They use it for some carry on bags that wont fit in there overhead area as there not full size jets. My plan was to place it there and ask one of the people doing the baggage to please be careful loading it, explaining what was in it, and lay some good old american cash on them as a little insurance.
While I was waiting there for a baggage person to walk up, one of the flight attendants asked me if I needed any help, so I explained to her what I was waiting on, then the damdest thing happened. She said bring it on up, I got a special place just for it, so I did. I'll end the story by saying my sons comp got a better ride to Edmonton than I did because she placed it in a first class seat, strapped it in with the seatbelt and away we went, lmao. This was a full size comp, I built his in a coolermaster case.
I agree with the above poster that different airports approach things differently, but my travel this past summer with my sons had no hitch at all.