You know all those plugins and stuff that you try to install to make the comments author's links dofollow? I think they're very good and sometimes you should just use them, it depends on what you want.
But there's another way of making you comments author's links dofollow, directly from WP platform.
This post it's not about wether is good or not to have dofollow on the comments but just How To and that's all.
(I beg for your mercy regarding my English skills, as you can see, they're not so great...)
Go to public_html/wp-includes/comment-template.php
Around line you should have this, or something like this:
$return = "<a href='$url' rel='external nofollow' class='url'>$author</a>";
Just remove the nofollow from there and the links become dofollow.
Result should be like this:
$return = "<a href='$url' rel='external' class='url'>$author</a>";
I've tested this with Search Status addon for Firefox, it seems to be working fine.