It is overclocked because the clock speed is 147Mhz. Stock clock speed is 133Mhz. So Alienware have raised the clock speed to achieve an overclock.
But because they have also enabled the Speedstep facilities in the BIOS also your system will down clock when idling to save power.
So when you use your system hard it will run at 3.100Ghz and when idling it will run at well below stock speeds (2.660Ghz).