What is the simplest possible configuration of Apache2 mod_proxy?

I think you are almost there try adding

ProxyRequests Off
<Proxy *>
    Order deny,allow
    allow from all
 </Proxy>
 ProxyPreserveHost On

to your VirtualHost definition

My working config(ubuntu 10.04):

<VirtualHost *:80>
  ProxyPass / http://www.domain.com:8080/sitename/
  ProxyPassReverse / http://www.domain.com:8080/sitename/
  ProxyPreserveHost On

  <proxy>
    Order deny,allow
    Allow from all
  </proxy>

</VirtualHost>

That looks good. Check your error log. Do you have both mod_proxy and mod_proxy_http loaded? You should also have "ProxyRequests Off" to prevent you from becoming a forward proxy