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:

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.