Another vote for the GS mini, of course!!!!
I just picked up a GS Mini this week and I cannot believe how good it is at fingerstyle. For fingerstyle, it is superior to everything in my collection save my 324ce, and it yields very little to that one. Because of its size, I understand why it is referred to as a travel guitar, but for fingerstyle in can hold its own against a number of full sized instruments.
In terms of size, I think it could fit in an overhead compartment approximately 60-75% of the time. I don't think it will work at all on some of the smaller jets. My backpacker, which isn't in the Mini's league with respect to sound, is very slim and compact and is a more versatile traveler in that regard. The backpacker is a perfect guitar to practice with and maintain my callouses while on vacation - if you can get past its tinny banjo like tone quality, it might be passable at a campfire, etc - but I wouldn't use it for anything more than that.