Mac OS X 10.5: Web Sharing - "Forbidden 403"

In what follows, substitute your short username for myshortname

Create the file
/etc/apache2/users/myshortname.conf
containing the following text:

<Directory "/Users/myshortname/Sites/">
Options Indexes MultiViews
AllowOverride None
Order allow,deny
Allow from all
</Directory>

Set the group and owner as follows:
sudo chown root:wheel /etc/apache2/users/myshortname.conf

Restart apache by switching Web Sharing off then on in System Preferences.