This really old hint has the basics for resetting a machine back to no users, so it boots to the Setup Assistant again. Things have changed in Leopard, so here's some new info.
This hint will probably be most useful for folks who configure machines for a living, but if you're selling or giving away your machine, or just troubleshooting login issues, it may come in handy. Here are the Terminal commands to delete a temporary (or lone) account in single-user mode on Leopard (in the following commands, replace username with whatever account short name you've created). Note that this must all be done in single user mode (hold Command-S during boot).
First, the usual stuff:
$ mount -uw /
$ rm -R /Library/Preferences/
$ rm -R /Users/username/
(Note that deleting /Library/Preferences/ is optional and in some cases may cause problems after pre-configuring some third-party software. However, I do this after simple configs or installs, e.g., install an OS, apply Software Updates, and then give it to the end user to create a fresh account.)
$ rm /var/db/dslocal/nodes/Default/users/username.plist
$ rm /var/db/.AppleSetupDone
While getting rid of .AppleSetupDone wasn't really necessary for OSes 10.2 thru 10.4, you do have to get rid of the Setup flag file in Leopard, or when you reboot, you'll get a login window and no working account to log in with.
Mac OS X Hints
http://hints.macworld.com/article.php?story=2007110800450816