My teenage daughter loves iTunes, DVD player, games and other Mac apps -- so much so that they can interfere with her homework, music practice, chores, etc. So I limit her access to addicting applications with a password.
I use Disk Utility to create an encrypted disk image that is large enough to hold the application, then copy the app into this new disk image. I then trash the original app and any aliases, and -- with the disk image open -- make any new aliases I want (and move them outside the disk image), and also drag the app icon to the dock. Then I close the disk image.
Clicking on any of the aliases brings up the password box. Either I type in the password for her, or I have the option to change the password at will if she misses an assignment. For example, she must bring a progress report home from school every week. No progress report means she isn't given the week's new password until she brings home the report.
In the case of iTunes, I just moved the application, but left all the libraries and music files in their oringinal location. iTunes found the music files without any problem and seems to access the preferences off the hard drive as well.
[robg adds: Guess I'll need to remember this one in 12 years or so when Kylie gets to that age! On a more serious note, this solution could cause difficulties at the time of system updates -- I would recommend moving the apps back into their default location prior to running any system update, given people's past experiences with updates and relocated 'core' applications.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20051031202403360