This is not widely documented, and I have only tested it in 10.5. I also rebooted my machine to restart all daemons after running the command. If you try running it in 10.4, you will have to test it as I do not have a Tiger machine around me to test this out.
Open up Terminal and run the following command:
sudo /usr/libexec/airportd en1 ‑ibss_admin 1
To undo this, simply rerun the code, but replace the ending 1 with a 0, and then all users will be able to create Ad Hoc networks.

