Because of comment spam multiple search engines have developed a net tag to be used. Spammers send comment and trackback spam because the number of references to a web site increases the value of their link. The new rel="nofollow" tag tells search engines to ignore the the link.
If you are a Movable Type user there is a simple plugin that implements this tag automatically into the links from user comments and trackbacks. There definition of simple coincided with mine in this case. Only two files are needed to be copied and no initialization script is required to install them.