TPM has a good article on some of the loopholes that could have been exploited, and also links to a more technical discussion of some possibilities, revolving around the UBIT (Unrelated Business Income Tax) on pages 40-48.

http://tpmdc.talkingpointsmemo.com/2012/07/romney-offshore-i...

http://www.law.nyu.edu/ecm_dlv2/groups/public/@nyu_law_websi...