I'm actually working on this now. I hope to have a solution in the next week or so.
As a quick fix for comment spam, I added a required checkbox on my comment page labeled "I am a human". It's worked well so far, but it's just a matter of time I'm sure (not that my blog is all that important anyway)
I detailed the quick fix here: http://blog.climers.com/post/2009/08/15/BlogEngineNET-Comment-Spam.aspx
I'll post my recapcha solution when I'm done.