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.