Caches are kind of a touchy subject. I have no information that they are, but I would not be surprised if Google was inexorably moving toward a 'no user accessible cache' policy. The reasoning is pretty simple, people push things they don't mean too, then they 'fix' it, but if the GoogleBot was in the area your 'fix' may not be as durable as you hoped. Recent examples are iPhone 5 leaks on carrier sites, mis-priced sales at Walmart.com, and support sites at HP for products that were going to exist but now aren't.
Clearly its not Google's fault that people screw up and they happen to look at the wrong time, but a strict interpretation of Google's mission would suggest that keeping bad pages 'cached' isn't part of it. And while the vast majority of cached pages provide a great service like when HN exposure overloads a web site you can still see the content, Google cannot practically fix every cache page where the owner doesn't want it cached (they might not even know it is) and then fix it. (Yes you can email them to have them spike the cache but how many people realize that?)
A couple of the newspaper types went after them claiming the cache was an 'illegal copy' but of course the page didn't say 'no-cache' (which would keep it out of most CDN's as well so its painful).
So the cache feature is useful, but not everyone thinks its a good thing. If they have annoyed Google enough, Google may have just said 'screw it, lets get rid of this capability for end users.' But again, that is just speculation based on seeing it get harder and harder to 'discover' that Google has a cached copy somewhere.
