Skin stylesheets should be prefixed with sweaver-

From http://api.drupal.org/api/drupal/includes!common.inc/function/drupal_add...

Note that Modules should always prefix the names of their CSS files with the module name; for example, system-menus.css rather than simply menus.css. Themes can override module-supplied CSS files based on their filenames, and this prefixing helps prevent confusing name collisions for theme developers.

I personally had a collision between default.css from the default skin and a file in my theme named default.css.