Yesterday, Google announced1 that it will use the HTML5
rel to determine authorship of a web page as it crawls the web.
To use this, you can write the following:
<a href="../authors/andrewjones" rel="author">Andrew Jones</a>
It should link to a profile page on the same website.
Furthermore, WordPress 3.2 will come with support built in when it ships later this month2. Another reason to upgrade quickly :-)
For more information, see the Google help page.