But like the iPhone and iPod, the battery is built in. So if it dies you can't simply pop in a new one; if the battery is broken, for all intents and purposes, so is your iPad.
Apple says they'll replace the entire iPad device for a $99 service fee if you qualify for a battery replacement. That means the battery's charge capacity has simply worn down naturally. It doesn't cover water damage, or a device that's been dropped or modified in any way.