Better still, get a universal adapter. For a hundred bucks or so, you can get a power adapter that will work on AC, in a car or on a plane. And by swapping the tip on it, you can use it to charge a cell phone or iPod. That's the one I keep in the bag. Belkin and Kensington each have their own models, and there's the iGo that Radio Shack carries (but they don't carry the tips for Apple).
But be careful with the tips. They're easy to break and not real easy to replace.
The only problem with univeral adapters, is you only get a "maintenance charge" on most notebooks, unless you buy a high end ($$) adapter matched to your particular notebook. They're great for folks like traveling salesmen, who charge up in the car in between stops, but they won't keep a battery fully charged if you're using the notebook. Because of the built-in limitations, they will also kill your battery life over time. (ie, they're a permanent "brown-out")
On a recent trip to Australia, the Auto/AC adapter I had stretched battery life from 3hrs to 8hrs, but I still had to shut down and recharge before the trip was over.