I wouldn't recommend an extra battery. They can run $100-$150, and since you have to shut down to switch, they can be a pain.
Being a fairly new laptop, you should be fine for an extended plane trip. I flew MCO to LAX and back non-stop last week. It was 5.5 hours to LA, and 4.5hrs to Orlando. Each time, my laptop battery lasted, and had battery to spare. (I have a Dell 700m with the extended battery)
Make sure to disable WI-FI, as that is a big power hog. Set your battery meter to MAX BATTERY. (Right click on the task bar icon to adjust power properties) Reduce the brightness of the screen to minimal. It may seem dim, but you'll get used to it. And running movies from the HD will use less power than a DVD player.
In addition to movies, I downloaded games to my computer to use. MSN and AOL games can be downloaded. They all have a 60 minute trial period, but there are over a hundred games to choose from. Download a bunch, and you'll have other stuff to entertain yourself with.