When you receive a call on your iPhone, double-tapping the Power On button will send the call to voicemail. A single tap on the Power On button will mute the iPhone and let the call ring out. While the latest iOS lets you do this from the screen, this tip allows you to send calls to voicemail without even looking at your iPhone.
Reference: osXdaily.com, Feb 2 2011
More tips: carleton.ca/iteam/tips