This patch is a bundle of the following:
1. A modification to drupal_add_css, that chooses an RTL version, if one exists.
2. RTL CSS versions of core modules.
See this short thread for a better description: http://groups.drupal.org/node/4055
NOTE: the CSS files will probably need to be modified and updated again, as past experience shows that they are modified throughout the code freeze. However, in this patch we aim at functionality of the drupal_add_css function, and more exact RTL CSS files will come in the following days.