I did this on a windows machine following the instructions I laid out in this post. However, when installing Apache on Ubuntu using the apt system, you end up with a pretty strikingly different Apache configuration than I was used to. I found an incredibly useful walkthrough written by Robert Peters  that I'll basically re-write here.

1. After installing Apache and Tomcat, install the Jk module for Apache:

   apt-get install libapache2-mod-jk

2. Create a file at /etc/apache2/workers.properties and paste in the following lines:

   #Define 1 real worker using ajp13
   worker.list=worker1
   
   #Set properties for worker1 (ajp13)
   worker.worker1.type=ajp13
   worker.worker1.host=localhost
   worker.worker1.port=8009

3. Edit your Apache configuration by adding a few lines to /etc/apache2/httpd.conf

   JkWorkersFile /etc/apache2/workers.properties
   JkLogFile /var/log/apache2/mod_jk.log
   JkLogLevel info
   JkLogStampFormat "[%a %b %d %H:%M:%S %Y]"

4. Next, edit Apache's default site. In my case, this was /etc/apache2/sites-enabled/usgin. If you haven't already messed with the default site, it will be /etc/apache2/sites-enabled/default.
   1. Delete or comment out the line that specifies the DocumentRoot.
   2. Add the following two lines right below the line you just removed:

   3. JkMount / worker1
      JkMount /* worker1
      

5. Enable the "Connector port" 8009 in tomcat by uncommenting the following line in /etc/tomcat6/server.xml:

   <Connector port="8009" protocol="AJP/1.3" redirectPort="8443" />

   Simply remove the <!-- before and the --> after the line to uncomment it.

6. Now, restart tomcat and then restart apache:

   /etc/init.d/tomcat6 restart
   /etc/init.d/apache2 restart