root@linux1:/etc/apache2#  a2enmod usermod; cat /etc/apache2/mods-avaiable/userdir.conf; ls -la 
This module does not exist!
cat: /etc/apache2/mods-avaiable/userdir.conf: No such file or directory
total 60
drwxr-xr-x   7 root root  4096 2008-03-12 18:02 .
drwxr-xr-x 110 root root  4096 2008-03-12 18:39 ..
-rw-r--r--   1 root root 24175 2008-02-04 13:15 apache2.conf
drwxr-xr-x   2 root root  4096 2008-03-12 19:22 conf.d
-rw-r--r--   1 root root   895 2008-02-04 13:15 envvars
-rw-r--r--   1 root root     0 2008-03-12 18:02 httpd.conf
drwxr-xr-x   2 root root  4096 2008-03-12 18:51 mods-available
drwxr-xr-x   2 root root  4096 2008-03-12 18:48 mods-enabled
-rw-r--r--   1 root root    10 2008-03-12 16:32 ports.conf
drwxr-xr-x   2 root root  4096 2008-03-12 17:44 sites-available
drwxr-xr-x   2 root root  4096 2008-03-12 16:32 sites-enabled

also:

conf.d/userdir.conf 

<IfModule mod_userdir.c>
        UserDir public_html
        UserDir disabled root

        <Directory /home/*/public_html>
                AllowOverride FileInfo AuthConfig Limit
                Options MultiViews Indexes SymLinksIfOwnerMatch IncludesNoExec
        </Directory>
</IfModule>