I am not aware of any cracks of this DRM.
The copistar program uses what are essentially images of the screen, to build a "scan" of the ebook, see
Now that my mobi problem is solved. This isn't cracking the encryption, but might still be considered circumvention.
Another recent thread of interest is
Porting or new Funtionality, since it includes a report of using a print feature (which isn't often enabled) to extract content.