Michael,
I was wondering why the most recent posts don't seem to be indexed. If you pick examples from the January list , you'll probably notice that the most recent posts are from Jan. 25: [1] indexed, [2] not indexed.
Maybe this is because the bot (if it ever returned after initial indexing) looked at [3] and didn't notice any change, so it didn't descend further. If this is the case and the bot returns daily or so, we should see some change soon because February is new on [3].
But I doubt that the bot ever came back after Jan 26.
You could try to submit a changed sitemap.xml each month, pointing to that month's message overview page and specifying a changefreq of 'daily' [4] for that page. Don't know whether Googlebot will take this into account though. But other ways such as the infamous revisit-after meta tag won't work at all, I was told.
-Gerrit
[1] https://mailman.uni-konstanz.de/pipermail/basex-talk/2011-January/001072.htm... [2] https://mailman.uni-konstanz.de/pipermail/basex-talk/2011-January/001073.htm... [3] https://mailman.uni-konstanz.de/pipermail/basex-talk/ [4] http://www.google.com/support/webmasters/bin/answer.py?answer=183668