As of Dec 14, the mappaint plugin has been included in the JOSM core and can
now be found at http://josm.openstreetmap.de/svn/trunk/.  For details see
http://lists.openstreetmap.org/pipermail/josm-dev/2007-December/000475.html

Apparently, this version doesn't even build anymore with recent versions of
JOSM:
http://lists.openstreetmap.org/pipermail/talk-de/2007-December/005798.html