Computers that ship with Mac OS X 10.3.5 may include a Migration Assistant that allows you to copy over the settings and data from your previous computer. If you selected the option to move applications to the new computer, the AirPort Express Assistant may not migrate properly.
The result is that AirPort Express Assistant won't launch.
To resolve the issue, simply reinstall the software from the AirPort Express CD-ROM. Then re-apply the AirPort 4.0.1 Update if needed.
This document will be updated as more information becomes available.