I have just downloaded the latest version of the scheme document. We will discuss it at the Savannah meeting.
The documents are in
ftp://ftp.pwg.org/pwg/ipp/new_PRO/scheme-19980922.doc MS Word 97
The document has the following major changes in it since the previous version in August:
1) support of the 'ipp' scheme is mandatory
2) a security parameter is added to the URL.
This document is not intended to be a final draft for IETF. Rather it is for discussion.