WordPress has this very easy and straight forward logic when it comes to RTLing themes. It simply checks for the direction of the language you're using in your WP installation, if it happens to be 'rtl'(Arabic for example) it includes the stylesheet rtl.css if it exists. This means that RTLing a WP theme is creating this rtl.css file. A detailed tutorial is coming up soon about RTLing WP themes

Louy who writes by the way at Ardroid, WordPress Codex and Global Voices wrote a Wordpress Plugin that creates this rtl.css for you. He called it the RTLer(well of course):

RTLer, is just a simple plugin that generates the RTL stylesheet rtl.css for you from your theme's style.css file.

Check the RTLer out at the WordPress Plugin Directory.

Thumbs up Louy!