Tom Ewing's paper ( http://stlr.stanford.edu/pdf/feldman-giants-among-us.pdf , datestamped Jan. 2012) says they've identified 1,276 shell companies, which is quite a bit lower than the 2,201 items listed by PlainSite.

(In his interview with Ira Glass, broadcast in July 2011, Ewing said there's "very close to 1,300" shell companies, http://www.thisamericanlife.org/sites/default/files/TAL441_t... , so I guess he's been working on this for a while.)