Since I figure I'm not the only person using Ubuntu and OS X 10.4.8 together, I figure I'll post my working config which is running on Ubuntu 6.06.1 and is accessible from OS X and Windows XP.

######

in the apache config:

<IfModule mod_dav.c>
DAVLockDB /var/lock/mod_dav/Dav_Lock
Alias /ical /var/dav/davroot
DavMinTimeout 600
<Directory /var/dav/davroot>
Dav On
Options +Indexes
AllowOverride None

AuthType Digest
AuthDigestFile /etc/apache2/davusers
AuthDigestGroupFile /etc/apache2/davgroups
# Finder.app v 10.4.6 won't mount shares with the following directive enabled...
# AuthDigestDomain "ubuntu-server"
AuthName "ubuntu-server"

Require valid-user
Order deny,allow
Deny from all
Allow from 10.1.0 192.168.0
Satisfy Any
</Directory>
</IfModule>

######

at a shell prompt:

sudo mkdir -p /var/lock/mod_dav
sudo touch /var/lock/mod_dav/Dav_Lock
sudo chown www-data:www-data /var/dav/davroot /var/lock/mod_dav/Dav_Lock
sudo htdigest -c /etc/apache2/davusers ubuntu-server someusername

#######

....and i think that's all....