An error in the longitude values for the CSM field has been discovered by Smerdon et al. (2010), "Erroneous Model Field Representation in Multiple Pseudoproxy Studies: Corrections and Implicatoins." J. Climate 23:5548:5554. The most straight-forward correction for those interested in using the CSM data was to privide a corrected gridbox location file, which appears here as "longlatfixed".  
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The main conclusions of Mann et al., 2007 with regard to the reconstruction of the NH mean and overall field using the regem method still stand. Additional RegEM tests have subsequently been published which do not contain this error. See Rutherford et al. (2010) "Comments on "A Surrogate Ensemble Study of Climate Reconstruction Methods: Stochasticity and Robustness", J. Climate, 23:2832, the supplemental information (Table S2) in Mann et al. (2009) "Global Signatures and Dynamical Origins of the Little Ice Age and Medieval Climate Anomaly. Science 326:1256:1260, and Schmidt et al., "A comment on "A Statistical analysis of multiple temperature proxies: Are reconstructions of surface temperature over the last 1000 years reliable?" by McShane and Wyner", Annals of Applied Statistics. This manuscript is currently (January 2011) available at http://www.e-publications.org/ims/submission/index.php/AOAS/user/submissionFile/8349?confirm=2d010a44