Hijacking the URL for filtering

WordPress also has a rewriting mechanism, it’s better to use that instead of doing it in the .htaccess file.

You can add your own rewrite rules to it. In your case we want to store the cat1 part in a separate query variable, so we also need to tell WordPress to allow this query variable (it uses a whitelist for safety).

The precise form of the rewrite rule depends on what http://domain.com/news/ is: is it a Page with a custom template, or something else? You should find this out (using my Rewrite analyzer plugin for example), and add this to the rewrite rule.

add_action( 'init', 'wpse16819_init' );
function wpse16819_init()
{
    // Base form, will probably not work because it does nothing with the `news` part
    add_rewrite_rule( 'news/([^/]+)/?$', 'index.php?wpse16819_filter=$matches[1]', 'top' );
    // Use this instead if `news` is a page
    add_rewrite_rule( 'news/([^/]+)/?$', 'index.php?wpse16819_filter=$matches[1]&pagename=news', 'top' );
}

add_filter( 'query_vars', 'wpse16819_query_vars' );
function wpse16819_query_vars( $query_vars )
{
    $query_vars[] = 'wpse16819_filter';
    return $query_vars;
}

Remember to flush the rewrite rules (save them to the database) after you add this code. You can do this by visting the Permalinks page in the admin area.

You can now access the wpse16819_filter variable with get_query_var( 'wpse16819_filter' );.