In an email update to its Kickstarter backers,
This SDK is a set of tools that will make it possible for developers to create and share their own watchfaces with Pebble users.
There was lots of speculation that Pebble hadn't released this yet due to a yet-to-be-revealed acquisition by Apple, but those rumors can be put to bed.
Check out the video below for elaboration on the SDK and a quick demo of the watchfaces in action.
Update 34 - Pebble Watchface SDK in April from Pebble Technology on Vimeo.