With the changes i recently committed to the OSM SVN repository osm2pgsql can now read OSM files in the new binary PBF format directly (with parsing code based on that found in pbf2osm). Files in PBF format are available from the geofabrik download site along with their XML counterparts and are usually about 30% smaller than bzip2 compressed XML. osm2pgsql also parses PBF about twice as fast, so substantially reducing the time taken for the first processing step.
Also part of my committed changes are an improved autotools setup that checks for library and header file availability in the configure stage already, and that only includes PBF support if the needed GNU ProtoBuffer related tools and libraries are available.
Unfortunately the PBF code requires at least protobuf-c 0.14, which is not yet part of current distributions, so you'll have to install that one from source yourself. But with proper configure checks in place you'll be at least warned about version mismatches here and osm2pgsql will be built without PBF support then.