How to add new attributes to the HTML tag - Sasson sub-theme template.php

To use the fb project I had to a new attribute to the HTML tag of my sasson 3.x sub-theme.

I tried to add a new function my template.php file, e.g.

<mytheme>_preprocess_html(&$vars) {
  $vars['html_attributes'] .=  '  xmlns:fb="http://www.facebook.com/2008/fbml"';
}

But it didn't work, maybe because this is overridden by sasson_preprocess_html() in sasson/includes/preprocess.inc, which defines the variable again.

Any idea how to do it right? (for the meanwhile the only thing which worked was str_replace on html.tpl.php).

Amnon