I carry my acoustic (a Baby Taylor, so it's a bit smaller than a regular acoustic size) all the time when I travel and it's no problem. It's in a soft case and can easily fit in the overhead. I normally carry on a small luggage, a hand carry bag (laptop) and my guitar and occupy half a regular overhead bin.
If you ask them nicely (at least on SQ), they'll keep it for you in a special cupboard where they put all the suit jackets, so no chance of it getting damaged.
<EDIT>
Just read SOFT's 'keyboard damaged in suit jacket cupboard' post, so clearly my 'no chance of damage' statement is a lie.