The world is afire this morning with talk of the announcements by Six Apart and the three major search engines (Google, MSN and Yahoo) to support a new HTML attribute named nofollow (in full, rel="nofollow"). By adding this attribute to your link anchors, the search engines will no longer consider the linking page as a component of the linked page's rank.
The idea is a simple one: Add this attribute to all link tags that you don't want to contribute to the rank of the linked page. For instance, comments and trackbacks, which are submitted to your site by users, are by their very nature considered "unknowns". Adding this attribute to those tags means you don't "stand up" for the value of the links. Very mafia-like. Joey Pants would be proud.
The first thing to consider is that this attribute is not easily applied to only particular URLs. If you have comments (or trackbacks) that you want to promote, and you want to exclude the nofollow attribute from them, it's not very easy to do. That isn't to say that that won't change in the near future, but it may require something along the lines of TypeKey to implement in anything like an automatic fashion.
Furthermore, remember that spam is a numbers game. Many people have spam filters of one sort or another on their email programs. Yet the spam still comes. The illustration is no different here. As Mark Pilgrim said (more than a year ago): "Spammers have it in their heads now that weblog comments are a vector to exploit. They don't look at individual results and tweak their software to stop bothering individuals. They write generic software that works with millions of sites and goes after them en masse.".
Removing even half of the potential avenues to this exploit still leaves a tremendous amount of options available, and expecting half of the avenues to go away is rather ambitious, to say the least. While this action might help Six Apart's reputation in the immediate term, as well as allow Google, MSN and Yahoo! to offer better search results, it is in reality not that much of a help to an individual site owner at this point. This will not stop spam coming to your site today or tomorrow. It may not stop it at all, as long as avenues remain unprotected, and according to the apparent leader of the space (Six Apart), "...there will always be some websites that accept user-submitted content without filtering it.".
This means you will not be relieved of the burden of spam on your site. For that, you need another solution. Perhaps MT-Approval. I use MT-Approval in conjunction with MT-Moderate, and I haven't had a comment spam in weeks. No removal, no adding to a list, nothing. It just hasn't been there. Maybe MT-SCode would help your site. Or MT-Blacklist. Or TypeKey. All are good solutions - and the nofollow idea isn't a bad one. Just realize that it isn't going to make all your problems go away immediately. This is one of those "invest for the future" ideas, and there is no telling if it will work, even then. Don't let your guard down in the meantime.
Update: Scoble has some interesting ideas about the use of the nofollow attribute. I think those make loads of sense. Fixing the world of comment spam? I don't see it so much. Once you remove offending comment spam from your site (which we've established you would need to do anyway), the link isn't there - so who cares if Google follows it or not - it doesn't exist!