At our Tucson meeting, James Kempf indicated that one important feature of the new version of SLP is that templates support inheritance. The template associated with one scheme (e.g., printer.ipp:) can thus inherit standard attributes from another more basic template (e.g., printer:) and extend it with the mandatory and optional attributes that are only relevant to IPP printers. I suspect (and in some cases I know for a fact) that a variety of printers (besides those supporting IPP and lpr) will want to use the printer: template (or a derivative thereof) to advertise their existence on the network.
Is this effort the definition of the basic printer template or of the IPP extensions to that basic template? I don't think they are one and the same as attributes that are mandatory in IPP may be optional or non-existent in other protocols and thus cannot be mandatory in the printer: template.