Is there any way to crack a password-protected disk image?
An article written by someone else who did that:  

http://blog.whitehatsec.com/cracking-aes-256-dmgs-and-epic-self-pwnage/

The author did get his cracked, but had to borrow a supercomputer to do so even after he got custom software designed to use partial knowledge of the password he had chosen.

It shows how hard a general cracking effort would be, but also how choosing a short or known pass phrase makes it practical in some cases to hire an expert to bypass even a good solid encryption scheme.