Do you mean that google showed its own 404 page? If so, it is caching your page, it's just got some embarrassing technical glitch. Possibly related to the "can't access your site" bug. I hope so, because I'd hate to think a major search engine has two concurrent and completely unrelated bugs.
If they weren't caching at all, the original SERP would say something like "no cached copy available" --or there would be no "cache" link in the first place.