Can’t speak for the 3G iPad Simon but the battery on the wi-fi version is good for a day’s work, I’ve seen 10 hours claimed which may well be true. Certainly 3G on the iPhone 3GS eats up the battery v where there isn’t a 3G signal or wi-fi.
The 6 feet (1.8m for those over 40) power cable available for the iPad is useful if the battery life isn’t enough, often at meetings there is power in the room but just too far away. And unlike laptop / netbook power supplies it doesn’t have a dirty big transformer half way. It also lets you use the power socket on East Coast 1st class if you have an aisle seat at a table and the person at the window seat isn’t using it, without intruding quite so much on personal space!