A few months ago, I wrote about the issues you will face with installing MySQL on OS X 10.5, Leopard. I am pleased to inform everyone, that this problem has been fixed!
The bug in question, mysql#28854, clearly stated that the problem was with the PrefPane. On Valentine’s Day 2008, Alfredo Kojima (of Workbench fame) fixed the problem, and uploaded a new PrefPane, to ftp://ftp.mysql.com/pub/mysql/download/gui-tools/MySQL.prefPane-leopardfix.zip.
This fixes an incompatibility with the default shipped PrefPane. The new PrefPane also detects if the MySQL data directory (/usr/local/mysql/data) has the incorrect permissions (and if so, one should fix it).
So all Mac users, your first order of business is to download an appropriate MySQL from the MySQL 5.0 Downloads, and then download the new PrefPane.
Happy starting and stopping MySQL now, via System Preferences. Note that MySQL Enterprise 5.0.58 has already got this fix implemented. The current Community release is MySQL 5.0.51a, and does require the new PrefPane (and it will be fixed in the next release).