After much discussion as to whether to add a fluid width option to Bartik (Drupal 7's new default theme) we decided to place a fluid width subtheme in contrib until Drupal 8.
Antonelli doesn't change any styles in Bartik, it just overrides the layout with percentages. It has fixed width sidebars and the extra regions such as the triptych (tri panel) and footer columns are fluid.
We'll probably use the usage stats from this project as a gauge to whether or not we need to include a fluid width option in core for Drupal 8.
UPDATE Dec 20th 2010: I just cut a release (RC1), so finally its well on its way to a full release!
Last updated 19th December, 2010 - 1:57pm