My setup matches w3mmode ikiwiki.setup exactly.
My doc/index.mdwn just has a line or two of plain text.
When I try to edit that page in w3m, it works fine until I push [Save Page].
Then I just get a page that only contains "403".
ikiwiki version is 3.20110715ubuntu1.
w3m is 0.5.3.
-- terry