We've been buying the X-24/X-25 ThinkPads and get around 4 hours of work out of them. Battery tech is getting better all the time. Most systems will stay in standby for days on a full charge.
I think IBM claims the battery will last 2-3 hours. Claims and reality are often different. Just remember the bigger the screen the more it will drain the battery. Maybe you should just get an MP3 player if you want to listen to music on a plane. I wish the airlines would embrace the power receptacles so we can just plug in our laptops (I even have an airplane adapter - but I don't usually fly first class).