I can confirm that the writable data directory (/var/lib/mysql) is also the default mysql system home directory in at least Hardy (mysql-server 5.0.51a-3ubuntu5.4), Intrepid (mysql-server 5.0.67-0ubuntu6) and Jaunty (mysql-server 5.1.30really5.0.75-0ubuntu8).
I can confirm that the writable data directory (/var/lib/mysql) is also the default mysql system home directory in at least Hardy (mysql-server 5.0.51a- 3ubuntu5. 4), Intrepid (mysql-server 5.0.67-0ubuntu6) and Jaunty (mysql-server 5.1.30really5. 0.75-0ubuntu8) .