Having Problems Installing 64-bit MySQL 5.5.8 x86 on OS X 10.6.6
Fixed it. Listening to /usr/local/mysql/support-files/mysql.server: line 256: my_print_defaults: command not found, I took a peek inside. Turns out this is the script that's called when the startup item or the preference pane want to start or stop MySQL. Unfortunately, it doesn't seem to work out of the box anymore.

The comments near the top of the script seem to know this, and give you a list of options to make the script work. I took the easiest and added the installation path to the basedir variable assignment.