The initial release of the Nintex Workflow 2013 SDK has been released today!
Dennis Kennedy is Nintex's Programmer/ Writer and he'll be providing more developer focussed content and code across the platforms, each week he'll be updating the SDK with new content and code, and publishing a weekly update. Make sure you follow him so you don't miss a thing!
Read up on his intro post here The Nintex Workflow 2013 SDK is (initially) released! and you can access the SDK here http://help.nintex.com/en-US/sdks/SDK2013/
(Update: The SDK has moved, and the link has been updated accordingly.)