Extra style in captions after 3.1 upgrade

I recently upgraded to Word Press 3.1. My captions used to be set to a 500 width using CSS. After the upgrade, they are now 510 pixels wide. I have literally gone through every trouble-shooting scenario I can think of and have come up empty-handed. Here’s the code:

<div id="attachment_49" class="wp-caption alignnone" style="width: 510px">

I found this by looking at the page source on my browser. Where is this 510px coming from? It’s not anywhere in the template, code, css or post html (in the wp editor).

Thanks for any help!