here is a link to the report summary issued on 2/25/11. it examines options if the supreme court strikes down the mandate. requested by sen. ben nelson. it points out that options are not desirable but feasible.
here is title of report
Private Health Insurance Coverage: Expert Views on Approaches to Encourage Voluntary Enrollmenthere is one article that indicates nelson is not abandoning the mandate
here is a nice summary of gao report from the Hill.