Wordpress, out of the box, is pretty decent at avoiding spam. It could be better, and there are a few plugins that will help:
3 Strikes Spam Prevention Plugin - Kinda like an email spam filter, calculating "spam scores" for new comments, and killing ones that pass a threshold.
Kitten's Spam Words 2.0 - kinda like MT-Blacklist, but better integrated into Wordpress.
I've added both, and it will be interesting to see how successful they are...
UPDATE: I just tried adding some comments to make sure good stuff got through, and bad stuff didn't. When attempting to add some spam, I was redirected to the FBI website. That's perfect, AND funny! Thanks, Three Strikes for Spam!!!