Update (12/17/04 08:57 EST): I have just received a report that this trick does NOT work using the Safari web browser. This is likely becuase this is not the "proper" way to use BASE HREF. I will be posting a javascript version soon.
I tried this about 5 hours ago, and it works. I have received ZERO spam since I implemented this -- and I usually get hundreds a day.
The trick requires a simple template edit and works by tricking the spambots into think that your comment script is somewhere else. So when they try to spam you, they get 404 errors. Everyone else can post fine as usual.
Steps:
1. In your Individual Entry Archive, look for the following:
<form method="post" action="<$MTCGIPath$><$MTCommentScript$>" name="comments_form" onsubmit="if (this.bakecookie[0].checked) rememberMe(this)">
and replace it with:
<base href="<$MTCGIPath$>">
<form method="post" action="<$MTCommentScript$>" name="comments_form" onsubmit="if (this.bakecookie[0].checked) rememberMe(this)">
2. Further down the page, after the </form> tag, enter:
<base href="<$MTBlogURL$>">
3. Rebuild your Indivdual Archives, and you are done.
This tricks spammers into thinking that your comment script is in the same folder as your entry. Since I starting using this, I have received zero spam and about 50 'Page Not Found' 404 errors in my web stats, presumably spambots trying to look for my comments script in the wrong places.
In combination with this, you may want to rename your comments script, just to be safe. I have not done so, but it may help.
This trick -- like many others -- is easy for the spammers to work around. If enough people start doing this, they will make adjustments to defeat it. For now, however, it is working extremely well. 
