iFixit

Battery: 3.7V, 2.45Wh, 660 mAh

That's about 16% less capacity than the 789 mAh battery in the new iPod touch. However, Microsoft promises longer run time than the touch for both music and videos.

Let's make up some fun units. Apple claims 30 hours of music for the touch, and Microsoft claims 33 hours for the Zune. That means the touch uses 26.3 milliamp hours per hour, while the Zune uses only 20 milliamp hours per hour. However, if you really care about getting the most for your milliamps, it's hard to beat an iPod shuffle (7.3 mAh/hour).