One issue people have run into with upgrading to OS X 10.5 (Leopard) is that the MySQL 5 package installer doesn’t seem to startup without a bunch of issues. This is especially true for the preference pane that basically just crashes when you try to start the service. The problem seems to come from an issue with how the installer sets permissions on the data directory within the mysql directory. I fixed this by simply taking ownership of this directory with the following command:

sudo chown -R username /usr/local/mysql/data

You may have to modify this based on your configuration, but the essential problem is that the ownership of this directory is busted. After making this modification, I’m able to startup the MySQL server from the command line or preference pane without issue.