Apple has introduced a new method to entering Recovery Mode. You can find instructions and a video demonstrating the new method here.
Press and hold the Sleep/Wake button, then drag the power slider to the right, to turn off your iPhone.
Press and hold down the Home button as you connect your iPhone to your computer using a USB cable.
Keep holding down the Home button until you see the Connect to iTunes screen.
You are now in Recovery Mode and can perform a clean restore of your device in iTunes.
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