I started updating my WPMU sites to WP3.1 and noticed that AddThis-plugin has developed a lot from the 1.6.x versions I'm currently using. Especially the Setup-page has been overhauled. Unfortunately, I can't anymore find an option to choose my own custom style which I've added to the
addthis_social_widget.php file into the array of styles at line 49:
$addthis_styles = array(...)
The Setup-page apparently doesn't take account the added styles anymore. In the old version they automatically appeared in the drop-down option called "Button style". I guess in the new version they should appear in the "Basic" section under the "additional style options" link.
Please take account that many ppl have added their own styles to use in blogs etc and fix this!!