Wordpress and RTL
WordPress's back end can handle Arabic just fine, with both the editor and the database itself. If you want the front end to display in Arabic you'll need to use/develop a theme that uses Arabic.
If your blog is Arabic-only, you can add a CSS class to the div surrounding the post in the blog template file that sets the text-direction to ltr. If you use Arabic and English, you can set an if statement to decide what direction each post needs to be based on a custom field you add to the post from the back end.
Use the Arabic WP installation and use this plugin: http://wordpress.org/extend/plugins/admin-locale/ to switch back to the English admin interface.