In Tiger, the OS's Installer is now queue-able. Under Panther, if you were installing an application with Apple's installer and tried to launch another install, it would give a message dialog and stop.
In Tiger, it goes through configuration, and when you click the Install button, it gives a message stating that it will wait for the current install to complete first. You can set up multiple installs and they will go in order.
[robg adds: Correction -- as noted in the comments, this worked in 10.3, too.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050511230444987