Before I get flamed on this, let me make it clear that I love WP and don't regret the migration from MT. But...
In over two years of using MT I've gotten just a light smattering of referrer spam. In less than one week using WP, I've been "blessed" with thousands of such lovelies. :-(
Is there a connection? Seems strange it would suddenly explode, but perhaps that's all it is...serendipity. I have to wonder, though, especially in light of my anticipated move to build quite a few client sites using WP as a CMS...am I opening Pandora's box here unwillingly?
As for combating the evil slime, blocking it from appearing in the logs is one thing, but apparently you can also deny it via various .htaccess methods (as I'm exploring here). Still, a lot of maintenance to keep up with, and that's something I never had to do running MT sites.
(PS: Want to mention *again* that I love WP before anyone misreads this post as a knock on WP...more that I want to understand if WP is an aphrodisiac to those-who-should-be-staked-and-tortured, otherwise known as referrer spammers. :-) )