http://www.faa.gov/passengers/fly_children/crs/

I used one that I borrowed from my mom for my three year old on some trips (FAA/NTSA approved). No problems on flights, and it was small enough to bungee-cord to a carryon (basically, it was an ergonomic booster seat), which helped a lot when we went through airports (this was a couple of weeks after 9/11, too, so traveling light was important).