After some discussion on #osm IRC, I started running the following command:

curl -L http://planet.openstreetmap.org/planet-latest.osm.bz2 | bzcat | osmosis --read-xml file=/dev/stdin --write-pbf file=planet-latest.pbf

After 15 minutes or so I have a 1GB (and growing) .pbf planet file. I'll probably generate these weekly and put them somewhere with a quick network connection. Stay tuned if you're interested in a planet-wide .pbf.