apache & mod_pagespeed + varnish question

I have the following architecture on the web layer.

Varnish <-> Apache2 (mod_pagespeed <-> mod_jk) <-> Tomcat
    ^          ^                                      ^
    |          |                                      |________: mod_jk
    |          |_______________________________________________: port 8080
    |__________________________________________________________: port 80

Where varnish and mod_pagespeed is installed with default config.

Now when i access the url http://example.org:8080 then i can see in the source code that mod_pagespeed was invoked and has rewritten the page (on the second refresh mind you as it wasn't cached the first time)

However when i access the url http://example.org:80 mod_pagespeed is not invoked and the html code is rendered unchanged.

The following shows the http headers recieved on each junction

PORT 80
< Server: Apache/2.2.14 (Ubuntu)
< Set-Cookie: JSESSIONID=95ABA0EB57ACCBE15C9D7035C4397BD3; Path=/
< X-Mod-Pagespeed: 0.9.17.7-716
< Cache-Control: max-age=0, no-cache, no-store
< Vary: Accept-Encoding
< Content-Type: text/html;charset=UTF-8
< Content-Length: 13234
< Date: Wed, 01 Jun 2011 12:17:11 GMT
< X-Varnish: 579414176
< Age: 0
< Via: 1.1 varnish
< Connection: keep-alive

PORT 8080 (mod_pagespeed working fine)
< Date: Wed, 01 Jun 2011 12:19:26 GMT
< Server: Apache/2.2.14 (Ubuntu)
< Set-Cookie: JSESSIONID=DC3759CDE52D3B63A785F9C97B005EC0; Path=/
< X-Mod-Pagespeed: 0.9.17.7-716
< Cache-Control: max-age=0, no-cache, no-store
< Vary: Accept-Encoding
< Content-Length: 15990
< Content-Type: text/html;charset=UTF-8   

Does anyone have any clues as of why this is happening? is it a bug in mod_pagespeed? is it caused by a config setting which is or isn't set in varnish or mod_pagespeed?

Any help to further drill down this issue is greatly appreciated.