My team uses a tool from Gomez Networks for load testing WordPress. We wrote scripts using their desktop application that performed basic actions like clicking a link to a blog post, posting a comment, etc... http://www.gomeznetworks.com/
The major drawback to Gomez is that it's expensive. You purchase units of Virtual User hours, and you can set up some extremely complex tests that simulate real world traffic. We had some pretty big issues early on with Gomez not being able to reroute domains through specific IP addresses. We have STAGING and QA environments that require hosts file modifications on a user's machine in order to access them, and a Gomez support tech had to pull some kind of magic to make that work in our scripts.
Alternatively, if you work at a large corporation, you may want to look at LoadRunner: http://www.google.com/search?aq=f&sourceid=chrome&ie=UTF-8&q=loadrunner.
And if you work at a small shop, check out WebLoad: http://www.webload.org/
I don't think you're going to find a tool specifically for load testing WordPress, unfortunately. It should be relatively easy to script out some basic blog interactions, like writing a post, posting a comment, moderating comments, searching, reading an RSS feed, etc...