The Righthaven case is slightly different, though I hope the logic still applies. (I'm not a lawyer, but I read the Righthaven opinion[1] when the Padmapper/3Taps workaround was originally discussed.) The difference is that Righthaven was granted merely the right to sue on behalf of the original copyright holder, but none of the exclusive rights that copyrights actually bestow upon their owners. It could be interpreted that posters grant Craigslist some of their exclusive rights ("copy, perform, display," etc.), and suing to protect those rights may be legally kosher. I know of no such precedent.

My personal interpretation/hope is that the right to sue for copyright infringement is nontransferable, which would give Craigslist no standing to sue. Individual posters could sue, however.

[1] https://www.eff.org/files/filenode/righthaven_v_dem/order6-1... (See also: http://en.wikipedia.org/wiki/Righthaven_LLC_v._Democratic_Un...)