I think this is why I use MAMP instead. Although I haven’t installed it on Leopard yet.
Upgrading to Leopard may break your web server
This evening, I finally found the time to write a small web application for the iPhone only to discover that the web server on my Mac was not working correctly. Every URL not returning a 404 in /~matt was returning a 403 Forbidden. After some searching and tinkering, I realized that my Apache configuration was not copied from Apache 1 to Apache 2 during the upgrade to Leopard. If you’re having this problem, you need to copy the files in /etc/httpd/users to /etc/apache2/users and restart the web server. If you made any changes, such as enabling PHP, to /etc/httpd/httpd.conf, you’ll need to make those changes in /etc/apache2/httpd.conf, as well.
2 Comments 
Oh, was this a perfect hint. You will see a problem if you do http://locahost/~username/ and you get the forbidden message. To do the copy you will need to
sudo cp /etc/httpd/users/* /etc/apache2/users
sudo apachectl restart
That will do it for the users. However, you can not just copy the httpd.conf file. The format has changed. ALSO, we are now using PHP5, not PHP4.
Enjoy!