- Enable support for Assistive Devices in the Universal Access System Preferences panel.
- Change the password string to the password of the user account you want to switch to.
- Name the script like the short username of the user you wish to switch to.
- Save the script as an executable, to allow easy double-click operation.
The script: For your convenience, I've also created an AppleScript application bundle version (32KB download), complete with icon. Just open like a normal Script in Script Editor (in 10.5 at least), put in your password, rename to the short user name, and enjoy the icon.
[robg adds: I haven't tested this one, and note that storing account passwords in a plain text file on your drive is at least something of a security risk. You could mitigate that issue somewhat by keeping your scripts on an encrypted disk image, though this will make using the scripts a bit less convenient.]

