WordPress 3.0 TinyMCE Editor Styling to Match Theme Style | Ed Nailor
Now you can control the styles of the WordPress content editor (TinyMCE) by using the following snippet in your functions.php file, and by adding an editor-style.css file to your theme.
add_editor_style();
http://www.deluxeblogtips.com/2010/05/editor-style-wordpress-30.html