WordPress uses Akismet spam detector to keep spam posts off of wordpress.com hosted blogs. Akismet is developed by the same crew that develops the wordpress software itself.
I’ve been blogging here since March 23rd, 2007. Since then, I have had exactly three comments show up in my “to be considered valid” comments queue that were likely spam (in fact, they were largely meaningless comments with an blog-linked poster name, so not exactly spam, just people trying to drum up a couple extra links)… and exactly one comment that showed up in my Akismet spam comments queue that was a real comment. There are 499 published comments on this blog at the time of this writing.
That’s a damn fine “false positive” and “false negative” rate for any spam scanner. I commend Matt and crew.