Ever since Movable Type started to have the comments redirection feature (since version 2.66), comment authors' URLs have been redirected via a simple redirection script that prevents them from appearing directly on blog entries. This (partially) solved the problem of comment spammers because links from comments no longer benefit from backlinks that add to their PageRank. However, legitimate commentors also have their URLs redirected, which to me seems just quite a bit unfriendly.
Neil Turner has worked around this with his aptly named Friend or foe hack for MT. A simple but still neat bit of PHP (and a good example of MT's excellent template tags). If you use MT and want to "reward" your legitimate commentors, you won't want to miss this.