According to Aaron Wall from SEO Book Google’s Cache date is the new PageRank.
Websites that are considered trustworthy and authoratitive are visited more regularly by Google. The Google cache data therefore reveals how much Google trusts your website by period of time before your website is revisited.
It stands to reason that we need to keep an eye on how long it is before Google bot revisits the website, this will give us a view of how google is rating the website in a smaller time period rather than waiting for the PageRank toolbar update every three months which gives us a broader inidication.