According to http://src.opensolaris.org/source/xref/pkg/gate/doc/facets.txt, facets should be identified with "facet.<facetname>=<value". Currently, pkgbuild.pl simply sets "<facetname>=true" for devel, doc, and l10n pkgs.
The attached patch sets the facet.<name> tag correctly for devel and doc. l10n is not mentioned in the docs, so I've omitted it.
pkgbuild-01-ips-facets.patch