Masscheck results indicate spammers have stopped abusing Google cache as a redirector about 3 weeks ago. It appears that previous redirects already in the cache still work, but perhaps Google changed their system to prevent future redirects from getting into their cache. We'll continue to keep an eye on this.
UPDATE: 1/6/2011 - now catches more variations
For the past month or more spammers have been abusing Google's cache as a link redirector. Normally if a spammer includes links in their message body, it is easy to identify that message as spam because the domain of that URI is listed in the numerous URIBL's. But by using Google cache as a redirector they often sneak past the URIBL's with an overall low score. Read more for the custom rule syntax and analysis.