This bug actually terminated by mysql server this morning. I do run unattended-upgrades daily by cron. This upgraded all mysql packages apart from the mysql-server-5.0 package to version 5.0.32-7etch8. I'd assume the real server package is tagged interactive or something so that it doesn't get touched by unattended-upgrades. Anyways, just wanted to mention that it's not quite as unlikely that mysql-server gets upgraded independently from mysql-server-5.0 as you assumed.
Here's the apt log of the automatic update: (Reading database ... 131272 files and directories currently installed.) Preparing to replace mysql-common 5.0.32-7etch6 (using .../mysql-common_5.0.32-7etch8_all.deb) ... Unpacking replacement mysql-common ... Preparing to replace libmysqlclient15off 5.0.32-7etch6 (using .../libmysqlclient15off_5.0.32-7etch8_i386.deb) ... Unpacking replacement libmysqlclient15off ... Preparing to replace mysql-client 5.0.32-7etch6 (using .../mysql-client_5.0.32-7etch8_all.deb) ... Unpacking replacement mysql-client ... Preparing to replace mysql-client-5.0 5.0.32-7etch6 (using .../mysql-client-5.0_5.0.32-7etch8_i386.deb) ... Unpacking replacement mysql-client-5.0 ... Preparing to replace mysql-server 5.0.32-7etch6 (using .../mysql-server_5.0.32-7etch8_all.deb) ... Stopping MySQL database server: mysqld. Unpacking replacement mysql-server ... Setting up mysql-common (5.0.32-7etch8) ... Setting up libmysqlclient15off (5.0.32-7etch8) ... Setting up mysql-client-5.0 (5.0.32-7etch8) ... Setting up mysql-client (5.0.32-7etch8) ... Setting up mysql-server (5.0.32-7etch8) ... -- To UNSUBSCRIBE, email to [EMAIL PROTECTED] with a subject of "unsubscribe". Trouble? Contact [EMAIL PROTECTED]