Attention Developers: we now have full API documentation available, as output by PHPDocumentor.
It's far from finished: it needs to be tidied up, better organized, and much of the code isn't well documented in the comments. However, hopefully it will be a useful starting point, and should grow over time.
We're still hoping to add some tutorials soon, specifically for those interested in how to use C5's model-view-controller syntax to build applications that make use of multiple block and page types.