Actually, I'm totally wrong. I thought this wasn't part of the original spec for some reason, but it's totally in Gruber's list of features and works in Markdown.pl, I just missed it when checking to confirm originally (I saw automatic links and missed the other links feature).

Thanks for causing me to look again (I used babelmark[1] to check what versions supported it and noticed all of them do, including Markdown.pl). I dislike putting out wrong information. :/

1: http://johnmacfarlane.net/babelmark2/?text=%5Btext%5D(url)