A petty annoyance on Ubuntu is getting all the bits of mailman to talk to each other: the mailman software itself, and in my case Postfix as the MTA and Apache as the host server.
Putting aside my complete failure to properly RTFA on the Mailman-Postfix hookup, I found an interesting problem with the default install of Mailman: if you modify permissions to allow Apache to reach the relevant cgi-bin directories, parts of Mailman stop working. Modify it back using the included check_perms script and Apache stops working.
The solution is to add the Apache user (www-data) to the Mailman group (list). Then everything works.