A WebmasterWorld thread reports that some users are seeing the URL webcache.googleusercontent.com being used for the Cache link in the Google search results. Typically, Google does not use a name based URL for the cache link, such as google.com or the one above. Typically, Google would use an IP address.
But some users are spotting webcache.googleusercontent.com being used instead of the IP address.
If you try going to webcache.googleusercontent.com it will actually load a Google search page. Google is the owner of googleusercontent.com and currently, webcache.googleusercontent.com's IP address for me is 22.214.171.124, which is the same IP I see being used when I click on a cache link in Google.
I have never seen this domain being mentioned before either. I guess it would make things cleaner to use a domain for the cache URL.
Forum discussion at WebmasterWorld.