You can do it directly with Windows 7.
You have to go in the "Power Option" menu, select "Edit plan settings", click on "Change advanced power settings" and "Processor power management" and put it at 80%... The frequency should change. 😉 You can see it with All CPU meter
It's too bad that we can't do it with OS X. 🙁
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Another thing, Mac OS X change almost every second the frequency of the CPU. For a Macbook Pro 15" 2011 with the 2.2Ghz i7, it goes from 792Mhz to 3.3Ghz. It manages itself the heat... But it would be nice if users could change the upper limits (From 3.3Ghz to 2.0Ghz for example) of the iX processors.