I am not a lawyer, but I do know a little bit about DRM. I believe the DMCA makes cracking all forms of DRM illegal except in certain cases which are outlined by the Library of Congress every three years. The last time the exceptions were updated was July 2010 [1], so they will be up for review again in 2013. If I'm interpreting the current guidelines correctly, you can only crack ebook DRM if it is preventing you from using text-to-speech (presumably for blind people or people with poor eyesight).
[1] http://www.copyright.gov/1201/2010/Librarian-of-Congress-120...