SmartTap Tweak enables You to Wake Your iDevice By Double-Tapping it

SmartTap is a new tweak from Elias Limneos that lets you wake your iPhone, iPad, or iPod touch just by double-tapping the screen.

You can also use other gestures, like slide up from the bottom or slide down from top of the screen while its off, and have it perform a different action, like unlock the device or launch a particular application, while the screen is off.

You can get SmartTap from Cydia Store for $1.99.