When backing up courses in 1.9.2 (and I believe earlier 19 versions) , even though I specify users to be included they are not.
This can be seen in the xml via text search.

Manifests itself as "no users found! " on restore of backup

I have space on 2 different commercial linux servers and can easily produce this problem, both with latest download and fantastico install:
Have also noticed on my mac osx installations.

create new 1.9.2 installation
create new course
add users
backup including users
try to restore, everything works but users.
expected users to be restored.

Restore of older backups 1.8.x with test systems DOES restore users

Also mentioned in forum on using moodle course at moodle.org by 2 other people
http://moodle.org/mod/forum/discuss.php?d=96791#p475638

I have only tested with mysql