HP moved a program that is referenced in some HP PPDs. If you are using the ExtremeZ-IP PPD download capability this is an issue if you need to support both Leopard and pre-Leopard clients with the same PPD.
The HP PPDs in Tiger reference a HP CUPS filter at: /Library/Printers/hp/filter/hppostprocessing. On Leopard this file has been moved to: /Library/Printers/hp/filter/hpPostProcessing.bundle/Contents/MacOS/hpPostProcessing. This means that by default the same PPD (and thus same EZ queue) can't be used to serve both Tiger & Leopard clients.
Fortunately if a symbolic link is created from the path referenced in a Tiger PPD to the actual location of the HP CUPS filter on Leopard, then things work fine. This link can be created using the following commands in the Terminal:
ln -s hpPostProcessing.bundle/Contents/MacOS/hpPostProcessing hppostprocessing