I've had problems starting up MySQL database server on startup on Snow Leopard.  MySQL preference pane seems to be flaky and it often fails to start MySQL server.

I created this startup script myself and it seems to work nicely.

sudo nano /System/Library/LaunchDaemons/org.mysql.mysqld.plist

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>RunAtLoad</key>
<true/>
<key>Umask</key>
<integer>7</integer>
<key>UserName</key>
<string>_mysql</string>
<key>Disabled</key>
<false/>
<key>WorkingDirectory</key>
<string>/usr/local/mysql</string>
<key>GroupName</key>
<string>_mysql</string>
<key>KeepAlive</key>
<true/>
<key>Program</key>
<string>/usr/local/mysql/bin/mysqld</string>
<key>Label</key>
<string>org.mysql.mysqld</string>
<key>ProgramArguments</key>
<array>
<string>--user=_mysql</string>
</array>
</dict>
</plist>

sudo chown root:wheel /System/Library/LaunchDaemons/org.mysql.mysqld.plist
sudo launchctl load /System/Library/LaunchDaemons/org.mysql.mysqld.plist
sudo launchctl start org.mysql.mysqld

Restart your Mac and your MySQL server should be running.

Marko