and one in which I've been personally involved on behalf of clients.
Introduced in 1997, the Google cache system is unique among commercial search engines. The system allows searchers to access a copy of almost any web page from Google's own servers, in the form it was in when it was last indexed. That could mean the page accessed is either minutes or months old, depending on when Google's robots last crawled it.
Google have been at the centre of controversy over this issue, based on the supposition that there could be copyright infringement involved in making unauthorised copies of web pages and then distributing them. Some site owners also expressed concern that the Google-cached version of their pages could differ from their server copies, and that any libel for instance, might remain in the cache for some time after it had actually been removed from the site proper.
A US district court judge has ruled that there's no copyright infringement however, saying that it is the user, by his/her act of downloading the cached page who creates the copy, and that such copying constitutes "fair use" under american law.
In the UK, the E-commerce Regulations protect search engines. They provide that a company like Google will not be liable in damages (or other remedy or criminal sanction) where the caching is "automatic, intermediate and temporary for the sole purpose of providing a more efficient service".
You can prevent your page from being cached by including a "NOARCHIVE" tag to exclude robots.