I've flown with a Cosco Scenera on E-145s, and they fit. If you have a bulky car seat, I recommend buying a smaller one (the Scenera is great for this) for plane travel, not only for making life easier for you when you install the seat, but also for hauling it through the airport. And as someone said above, make sure it's FAA certified.
Also, make sure to practice installing the seat with a lap-only belt. It can be a bit of a pain on airplanes, but once you get the hang of it, you're good.
Finally, if there's a lot of airport transitioning, I highly recommend a car seat trolley. You strap the seat to it, and push seat+toddler comfortably through the airport. They fold small enough to fit almost every overhead compartment.
[Just realized this is an older post -- hopefully above will be useful for others.]