BUD switch

The BUD Switch is a block planned to be added in the Redstone Update for use with redstone circuitry. BUD stands for Block Update Detector and configurations of such a device have existed in Minecraft due to a glitch labelled as a "feature", allowing circuits to detect block updates, such as growing grass or decaying leaves.

Trivia
BUDs are currently redstone circuits which range from very simple to very complicated. This block, which could be added in 1.5, will make them all very simple, only consisting of 1 block.