Ok, I did it :). Made a patch also. (http://pastebin.com/028wDbJR)
Changelog:
I've modified api.update_page() to save the new diff information.
I've added a Page.Revision.Diff class for storing the diff text and revto id.
I've modified site.loadrevisions() method to support rvdiffto parameter.
A method from Page.py is still missing to get diffs just like you get a revision now. But you can get the diff text from page._revision[id].diff.text
Please review the ugly as hell patch that I made. Any comment is welcome !.
--Matias