I know this is an advertised benefit of Windows 7, but now I can testify first-hand to the better battery life. I bought a laptop with Vista on it, and upgraded it to 7 today. I charged the battery before installing, and unplugged it soon after it started installing.
I haven't plugged it in yet. I have been using it for about 2.5 hours now, and the battery is at 73%.
Granted, it is a very good battery, but if this had been Vista, it would be at about 20% by now.
Windows 7 is a great OS, isn't it?