I've got the Dec '11 SE data dump downloading and there appears to be many sites missing.
A (potentially incomplete) list of the sites that appear to be missing - there are simply no files in the torrent for them:
It looks like no sites that went beta post-September made it into the dump, and a selection of sites that were included in the last dump are also AWOL.
These will be included in the next data dump in three months from now.
UPDATE: After checking the torrent it appears we were cut off in the middle of the upload process to ClearBits for an unexplained reason. We have posted two updates to the data dump which should include all of the missing or corrupted sites:
http://www.clearbits.net/get/1884-dec-2011-update.torrent:
superuser.7z
http://www.clearbits.net/get/1885-dec-2011-update-2.torrent:
bicycles.stackexchange.com.7z
cstheory.stackexchange.com.7z
dba.stackexchange.com.7z
drupal.stackexchange.com.7z
meta.bicycles.stackexchange.com.7z
meta.cstheory.stackexchange.com.7z
meta.dba.stackexchange.com.7z
meta.drupal.stackexchange.com.7z
meta.physics.stackexchange.com.7z
meta.security.stackexchange.com.7z
meta.tex.stackexchange.com.7z
meta.unix.stackexchange.com.7z
meta.webapps.stackexchange.com.7z
meta.webmasters.stackexchange.com.7z
meta.wordpress.stackexchange.com.7z
physics.stackexchange.com.7z
security.stackexchange.com.7z
superuser.com.7z
tex.stackexchange.com.7z
unix.stackexchange.com.7z
webapps.stackexchange.com.7z
webmasters.stackexchange.com.7z
wordpress.stackexchange.com.7z
