I have the tag plugin enabled, and these additional lines in my setup file: 

    tagbase => "tags",
    tag_autocreate => 1,
    tag_autocreate_commit => 1,

However, when I use a !tag or !taglink directive, nothing gets autocreated in the tags/ directory. What am I doing wrong? 

Edit: I'm using ikiwiki version 3.20100122ubuntu1 on Ubuntu 10.04.3 LTS... upgraded to ikiwiki_3.20110905_all from the debian repository and that solved my problem. Oops. :)
