It looks like there may be a solution for comment spam on the horizon. The search engines are adding support for an attribute that, in effect, tells the search engine spiders to ignore certain links. The hope is that this will destroy the utility of comment spam, but only is as many people as possible implement it
The search team at Google approached us with the idea of flagging hyperlinks with a rel=“nofollow” link attribute in order to alert their search spider that a particular link shouldn’t be factored into their PageRank calculations. The Yahoo and MSN search teams have also indicated they’d support this new spec, and we’ll be implementing and deploying this specification as quickly as possible across all of our platforms around the world.
It sounds like a good idea to me. You can download a plugin for Movable Type, Typepad and LiveJournal will have support shortly. I imagine the other blogging tools will be following shortly.