AnandTech got their hands on one of the new Jasper revisions of the Xbox 360 console. They discuss the changes, show you how to identify this revision, dissect it and investigate the power consumption. You can check it out over here. The reporter confirms Jasper runs cooler and says it uses about 12 percent less power under load, and 8 percent less at idle.
Overall you're looking at a 12% reduction in total system power under load and under 8% at idle, definitely not the ~30 - 35% drop we saw with the Xenon to Falcon transition, but not insignificant either. Remember that the smaller your transistors get, the more leakage current becomes a problem; while your transistors use less power, they also tend to use more power when they are in a logical off state than they should. There have also been a number of advancements in architecting for low power designs over the past couple of years that Microsoft hasn't taken advantage of as they would require a redesign of the 360's CPU/GPU. Microsoft is in full blown cost savings mode with the Xbox 360, the only things that will be done to that console before it dies are things to improve the Xbox division's bottom line.