It seems as if Google has switched the way it handles 301 redirects with the way it handles 302 redirects. According to a post by Ammon Johns at Cre8asite, he says:
The biggest change is in how Google is handling 302 redirects. It used to list nothing but the url, and have nothing cached for a URL where there was a 302 redirect. Now it is caching the page that the redirect points to, from the url where the 302 redirect is. But it now seems to have messed up on 301 redirects. Its like they got it backwards.
For an example of this check the cache of the URL for Overture.com http://www.google.co.uk/search?q=cache:www.overture.com/
http://www.google.co.uk/search?q=overture Note that google is now showing a snippet, not just a description for the Overture root domain. It also has a 'cached' link.
There are two threads, that I found, discussing this recent anomaly. One at Cre8asite named How Google Handles 301 and 302 Pages and an other at Search Engine Watch named Accidental PageJacking increases with new 8 billion index.