A Google Reader Help thread reports an increase in the number of RSS feeds being hacked into and used to redirect subscribers to spammer sites.
Googler, Roger, from the Google Reader team said:
Thanks for reporting this problem. Unfortunately, we've seen similar problems with WordPress feeds being compromised in the past. We'll look into it further, but in the meantime, I encourage you all to alert WordPress to this issue via their support forums.
Roger seems to believe there is a loophole somewhere in WordPress allowing this.
Clearly, this is not only upsetting to the publishers and their subscribers, but also to search spam. Many RSS feeds are scraped and used to inject content into Google. Spammed feeds that are scraped are can be even more of an issue for search quality.
Forum discussion at Google Reader Help.