At an event in San Francisco yesterday, Wintel claimed that upcoming processors, and Windows 7's improved power management will provide longer battery life, and better performance in certain programs.
They demonstrated power drain by playing a DVD on two identically configured ThinkPad T400s: one with Windows 7 (15.6 watts), the other with Windows Vista (20.5 watts). That could translate to about 1.4 hours of increased battery life.
The improvement comes through "timer coalescing", which lets one processor core sleep as long as possible if it's not needed.
The big gains should come when Intel's Arrandale (laptop Core i7/i5) chip hits later this year, possibly with Windows 7 on October 22. The dual-core processor (based on the 32nm Westmere shrink of Nehalem) is able to execute two threads per core.