There's one new features in 1.0a1: automatic links.
If you wrap a full URL in angle brackets, like this:
<http://example.com/>
Markdown will translate that into:
<a href="http://example.com/">http://example.com/</a>
This should probably work with email addresses, too, like this:
<gruber at example.com>
But doesn't at the moment.
-J.G.