i am a relatively frequent traveler (currently abroad at the moment). i recommend putting the mbp in a hard case, in a sleeve (g-form looks like the best), inside a backpack (or your carry on luggage). be ready to take it out when you go through the scanners, but they ought to let you leave it in that sleeve. with the sleeve and hardcase, you've got nothing to worry about.
i would never pack it in a suitcase. have you seen how the guys toss those things? also, tsa often "loses" items when they go through your stuff. usually not expensive things, and i suspect they are just misplacing books and the like, but annoying nonetheless, and no reason to take chances. there are reports of ipad thefts and the like. they are human, so i get it, but i am not keen on paying the govt. (taxes) to hire people to go through my stuff and steal it.
anyhow, a 13" ought to fit with its sleeve under the seat in front of you (take off and landing). that's great. it should be safe there. i doubt a 15" will, and you will have to put it in the overhead bins. the overhead bins are sites of much loss and destruction. people put water bottles up there to spill on things, they drop other people's stuff on the floor, and they shove things in there (crushing your stuff in the process). avoid that if you can, and keep an eye on it if you can't.
oh yeah, be careful when using it on your tray. people suddenly lean back and that can cause trouble if you have your lid wedged against the seat. usually it just rudely closes the lid halfway. i used to have many choice words for the #$%&! in front of me who dove backwards like that, but after a few years, you get tired of trying to educate everyone in the world about (un)common courtesy.
enjoy your trip!
