[ The following is derived from Tim Norman's original web site, and is useful background material for project historians - ajv ]
HP's decision was to develop drivers only for Windows 95 for this printer.
Developing drivers only for Windows 95 wasn't a terrible decision; the terrible decision was that PPA should be a closed protocol. That is, HP will not release any information about the nature of the protocol. Several of the core members are members of HP's peripheral developer's program, and even then, HP would not give access to the PPA specifications!
So, Tim Norman being a protocol hacker at heart, went forth and began to derive the elements of the PPA protocol by looking at the binary files produced by Windows 95 (when printing to a file). Tim's work was aided quite a bit by the discovery of several articles in the Hewlett Packard Journal. Particularly, the articles in the June, 1997 issue were quite informative.
After months of intermittent hacking, testing, coding, and numerous calls to HP (all of which were fruitless), Tim derived a working understanding of the protocol. This allowed me to write several programs, including a PBM to PPA converter. It is mainly intended for use directly with ghostscript, which produces PBM files in exactly the format the program accepts.