Just recently, I ran across the third Wordpress weblog in my feed list that had been hit with spam via what I assume to be the vulnerability fixed in version 2.3.1. It only shows up in feed readers, because it uses CSS to hide itself on the regular pages. That CSS is stripped by most feed readers' sanitizing process that removes all markup that may be malicious.
The striking thing about it is that all of the weblogs were related to web development: one was on a personal browser developer's website, one was a prominent web development news site, and the most recent one was the official weblog of a web browser. Now, I'm not necessarily putting the single browser developer at fault, since web applications aren't necessarily his area of interest. His webhost should make sure that classic security holes (like PHP's register_globals option) are turned offor disabled. On the other hand, the other two sites should know better. The web development news site has a significant number of posts on web application security, and the browser vendor deals with the security of its product every day, so surely they should be monitoring (or at least, find an automated process to monitor) feeds such as the ones at the National Vulnerability Database, in case exploits are discovered for any web applications that they may have installed.
To everyone else, if you can, please make sure that your webhosting environment is properly secured. Also, definitely subscribe to the news feeds of all the web applications that you run, because more often than not, there will be security vulnerabilities discovered, so you should upgrade as soon as possible in those cases.