I have flown with my Jack Russell Terrier a few times. He is 16 lbs, and does fit into a sherpa under seat bag, but can't stand up. He doesn't mind though since he can turn around just fine. No one has ever commented about him not being able to stand up, so I wouldn't worry about that.

He actually loves his little carrier since he knows it means he gets to go somewhere. We use it in the car as well since otherwise he is bouncing all over the place and it isn't safe for him or for me.

One thing to remember, the dog carrier counts as one of your carry on bags, even though they charge you extra for it... grrrr....

for going through security, you do have to take them out of the carrier and hold them going through the metal detester... but they do let you have a leash on them (which I do just in case he decides to make a dash for it!) and I have had TSA agents give me a hand getting him back into the carrier... there was just too much going on and to look at for him to go back on by choice.

Good luck. I am with everyone else here on not wanting to put them in as cargo. I am trying to figure out how I will move the dog if I end up moving overseas for work... that will be a long flight (or a couple of long flights)....sigh....