AnandTech has written a test report of Microsoft's new Xbox 360 Slim, you can check it out over here. The latest revision is more compact than the previous Xbox 360 consoles and features updated hardware that consumes a lot less power than its predecessors. The higher efficiency results in cooler operation, and makes the Slim less noisy than the previous Xbox 360 generations.
The biggest improvement is actually when the system is totally off. The new Xbox 360 slim pulled 0.6W compared to over 2W for the older 360s while doing nothing more than being plugged in. Idle power is roughly 75% of what it was with the Jasper Xbox 360 and load power is around 80 - 83% of what we saw with the previous generation. Note that the new Xbox 360 consumes less than half the power of the original 360!
Idle power consumption is actually not very impressive for a 40nm Xbox 360 if that's indeed what we're looking at. A modern day Core i5 system with an efficient power supply will idle at under 70W with a beefy discrete graphics card. While this is a significant drop compared to previous Xbox iterations, it's not impressive as a computing device.