I thought there were still serious problems with NaCl on x86-64.
http://groups.google.com/group/native-client-discuss/browse_... gives a brief overview, and there's been no word since that post, which was just over a year ago. I believe the problems have to do with NaCl's dependence on the segment registers; if I recall correctly, the APIs for manipulating them are the ones that were dropped in 64-bit Windows.