A New York Times article on a third-party group supporting John Edwards includes a great quote on the legal definition of “coordination” (which is prohibited):

Legal experts say the restrictions on coordination between campaigns and third-party groups are narrowly defined and difficult to apply.

“The definition of ‘coordination’ has been one of the most difficult legal concepts for the F.E.C. to grapple with for years and years,” said Kenneth Gross, a veteran campaign finance lawyer. “I don’t know if my wife and I met the standard for coordination before we decided to have a child.”