At around 10AM (EST) this morning, I was notified by Todd Malicoat that the Google cache server was down. I waited and then checked again, and it was still down. I started a thread on this topic at Search Engine Watch Forums at 10:08 AM.
Try it out yourself by doing a search on google cache or whatever you want. Then click on the cache link. What I have noticed is the 64.233.161.104 is having issues, but the 66.102.7.104 is fine.
WebmasterWorld reported this a bit later at 10:32 am. Danny blogged about it at 11:04 AM. So its still an issue at 12, noon.