I noticed the WP to WP importer not working right when mapping authors, and it turned out to be a bug fixed in WordPress rev 4735: http://trac.wordpress.org/changeset/4735
However, it is not merged to WPMU. If you are having the same issue. Add the patch in your WPMU.
Opened a new WPMU ticket:
http://trac.mu.wordpress.org/ticket/358