It is interesting, it seems that comment spammers cache the pages they spam - or read them from a cache, Googles perhaps? - because it has been a while since I removed the link-field from my comment-form, but would-be spammers still submit it.
I wonder if rot13'ing the input field names would be enough to throw off their harvesters.
So far I do three things that have proven quite effective (in combination): 1) The form goes to a preview-page, this stops a lot of comment-spammers, 2) Check whether the text-field has the same length when submitted first (preview) and second (save) time - apparantly the spammers that figured out to submit the second form aren't that good at interpreting the hidden fields correctly and 3) Don't allow any text that contains [[URL]] and http:// in them.
The last one sucks, obviously. But I think it is better than a CAPTCHA.