I'm not really sure why it was added in. I'm sure Rick had his good reasons. But an option would be nice.
This link in a post is appended with rel="nofollow", as are links in sigs.
If you use Firefox, my favorite feature is "View Selection Source" -- highlight some text on the screen, View Selection Source, and it'll show you the HTML just for that section. Try it on the links and you'll see it.
bbcode.inc.php can be edited to remove nofollow from links in posts, but now sure how to disable it for sigs just yet.