I've published the first part of the developer documentation for the 0.3 version of casetta on the web site: http://casetta.tuxfamily.org/developers/howto
I'll publish the last two parts soon.
I hope I'll not have to make a lot of change in that is described in this documentation until the release...
(If some code didn't work with the last svn version or some explanation are not enough understandable, please tell me it!)