1. Mount the disk image containing the HP scanner software.
2. Control-click to "show package contents."
3. Navigate within the package to a folder called "sub installers." There will
be a series of installation apps in there, all of which work in Leopard. Run
those, one at a time, and then you should be set.
I was able to make my HP Scanjet 3970 work with Leopard.
In addition, in the installer provided, there were scanner drivers for the HP Scanjet 2400, 3670, 3690, and 4070.
You can find those drivers here on HP's website.
Not all installers have this solution, but it is good to know when they do work.