I am not sure why there are so many different solutions to start xampp at boot. I had the same question, after reading all these solutions to no avail, they confused the hack of me. After a bit more trail and error, the answer is so simple and obvious. Open Script editor, type in this line:
do shell script "sudo /Applications/xampp/xamppfiles/mampp start" password "YOUR ADMIN PWD" with administrator privileges
Save it as an application. Go to System Preferences->Account->Login items, add this app to the list. Voila, once you reboot, Your xampp is served.
It works on both my Mac Tiger 10.4.11 and Leopard 10.5.3. I love xampp