Interested people may want to sign up for the development list, see http://lists.planetplanet.org/mailman/listinfo for more information.
Planet is available from Arch: The archives are:
jdub@perkypants.org--projects
    http://www.gnome.org/~jdub/arch

scott@netsplit.com--projects
    http://www.netsplit.com/arch
and the branch in both cases is: planet--devel--0.0
The current codebase is available from http://www.planetplanet.org/planet-devel.tar.bz2