Via Boing Boing and News.com, interesting news that a case saying the Google cache violates copyright has been ruled in Google's favor. Since Google makes it possible to prevent it from showing a cached page, the court ruled the publisher should have used that. In short, it you don't block caching, Google and other search engines have an "implied license" to reproduce your material. More in the court documents on the EFF site here (PDF format), and an EFF write up is here.
Postscript: Caching Made Legal - Do You Agree? I Don't! at the Search Engine Watch Forums has more analysis of this by me and some of the major concerns it raises. Read more or comment yourself over there. There's also excellent discussion at WebmasterWorld here.