Paul S. Ryan (FEC Program Director and Associate Legal Counsel, The Campaign Legal Center) has published 527s in 2008: The Past, Present, and Future of 527 Organization Political Activity Regulation, 45 Harv. J. on Legis. 471 (2008).  Here is the abstract:

In 2004 and 2006, political organizations with tax-exempt status under section 527 of the Internal Revenue Code spent large sums to influence elections without registering as political committees or abiding by the campaign finance regulations that go along with political committee status. Although many of these organizations later paid substantial fines to the Federal Election Commission, the FEC has yet to offer adequate guidance about which 527 organizations are required to register as political committees. As a result, many organizations are likely to attempt to influence future elections without registering as political committees, and to claim in later proceedings that they were unclear about their status. This Article analyzes federal and state campaign finance law, court decisions, and FEC settlement agreements to clarify what factors determine whether a given 527 political organization must register as a political committee. The Article proposes legislation to institute a presumption that groups which identify themselves as political organizations under section 527 are political committees unless they fall into a limited range of exceptions.