I am using Google Page Speed Service on http://www.minterest.com/ and deactivated W3 Total Cache. But Yoast's SEO guide (http://yoast.com/articles/wordpress-seo/#speed) says W3 Total Cache plugin + CDN will do magic.
I'm stil not sure about Google's PSS and whether to use W3 TC plugin again.
Google's response was
"We don't have experience with the W3 Total Cache plugin but from its description it sounds like it has only a subset of the functionality of PageSpeed Service, so it may be redundant. We're not aware of any conflict when it is used in combination with PageSpeed Service."