Check out the Readme for more information and grab the file DuetUpdateMay08.dmg from this directory. (Main website hasn't yet been updated).
From the readme...

From the readme...
So it looks like Apple finally fixed it and Apogee have the all-important driver ahead of 10.5.3. Happy days.The update is comprised of two parts, the Duet Software Installer and the Apple firewire audio driver v.242-4-7. The Apple firewire audio driver is for use ONLY with Mac OS X Leopard.
To Install
Download the Duet Update May 2008 file.
Double-click on the Duet Software Installer icon to initiate installation, and follow instructions provided by the Installer.
After installation of software it will be necessary to re-start your Mac.
Mac OS X Leopard only
Once the Mac has re-started, double-click on the Apple firewire audio driver v.242-4-7 icon to initiate installation, and follow instructions provided by the Installer.
After installation of the driver it will be necessary to once again re-start your Mac.