Here's the file I am using: http://home.austin.rr.com/dmcknight/xampp.zip
Feel free to download it and look at the parts with bbedit or a text editor. It contains a folder called 'xampp' with:
xampp (text file containing the startup command you'd usually use in the terminal)
StartupParameters.plist (text file that sets xampp to startup last)
--Localizable.strings (text file with the English text that is entered into the system log when xampp is started up)
I made this by copying and modifying a different startup item. Its set up to start xampp as the very last thing at startup. I have found that if I start my browser too early during startup, xampp will not be on yet.
To use this, you unzip the folder, and you need to change the permissions of the 'Resources' folder to your 'system'.
Then place it into your main root Library in the startup items folder.
Hopefully it will work just fine from there, but you might have to change the path in the file named 'xampp' if you have moved xampp somewhere other than the Applications folder.