Intel talked a bit about Core i7's new Turbo Boost Technology over at the IDF. TweakTown has some coverage and a video.
Turbo Boost works by dynamically adjusting the clock speeds of each core depending on what type of application is being used.
For instance, if you are using a multi-threaded application, all four cores will stay switched on and run at 100% clock. If, however, you are only running an application which is single threaded, all of the cores besides one will be switched off and that single core will get a speed boost with a higher clock speed.