I'm using Samba 3.0.14 with tdbsam backend. The modification timestamp of passdb.tdb does rarely get updated when any changes are made to the passdb (I guess this is only done when the file size grows, but I can't prove that). I'm sure there are changes to the file: I created a copy and changed a password. Running diff on the two files showed a difference. However, the timestamp was untouched. This is a severe problem with incremental backups: I recently restored a totally outdated version since the backup software never recognized the change :-(
*** This bug has been marked as a duplicate of 3101 ***