Yes, I actually already had a NeoCase for my iBook 14". I guess I'll just use that. I thought they might insist on it going "in the buff" so to speak.
Nate, it depends on the airport, but usually they will want you to take the laptop out of your carry-on, then put it in a plastic tray, separately from your other stuff (shoes, coat, coins) and then they pass it through the X-ray machine. In the old days they would also want you to turn it on to prove that it works but they don't seem to do that any more. I would leave it in sleep mode, though, just in case.
Sometimes a security type will stop you on the other side of the X-ray machine and ask you if they can pass an explosive-detector wand over the laptop. This doesn't affect it at all, so of course say yes (or risk a night in the pokey.)
A large part of why I actually wanted to buy a smaller laptop as opposed to the G4 14" is that you can barely even get the lid open to 30 degrees on an economy tray. Here's hoping this time it's first class or at least I can open the MacBook enough to see the screen.