We got a chance to play with the Belkin TunePower portable Zune battery, and found it's actually pretty handy for people who travel. The TunePower is essentially a large battery that latches on Yoda-style to the back of your Zune—except Yoda didn't stick his charger into Luke's dock connector hole.
The battery provides about 6 to 12 hours of extended play for your Zune, and our own play times fell between there. It's $59.99, which is kinda pricey, but worth it if you forget to charge your Zune or you go on long trips often.
The setup does add considerable heft to your Zune, making it pretty much impossible to pocket unless you're a clown or a member of Hammer's posse. Nevertheless, having something to listen to outweighs any bulk disadvantages when you're stuck on a plane.
Product Page [Belkin]