Something Can Be Done About It
I just noticed that WordPress comments are marked as “rel=external nofollow”. I got curious as to what that meant for SEO, as a tactic for SEO’s (and for spammers) has commonly been to deluge blogs with comments, hoping to thereby increase link popularity.
The best explanation I’ve found for this is on the Microformats Wiki, which explains the subtle differences between “nofollow”, “external” and the other “rel=” attributes.
In terms of what actual value and/or impact this would then have on the SEO value of comments in blogs, this is another one that’s a bit up to speculation.
One can first-off point out that an “external nofollow” tag would thereby mean that a link is meant to be marked as an external link that should not be followed and cataloged by any search engine.
However, there’s been rumblings all over the net that Yahoo! and Bing don’t give a damn about nofollow links, and index them anyhow.
So, unless you’re in the know and actually work for one of these search giants, chances are you’re going to have to rely on making your decision the old-hat SEO way – by empirical observation – and then determine what the impact really is.