Forcing reload of editor-style.css

There is a hook for that: 'mce_css'. It is called in _WP_Editors::editor_settings() and you get all loaded stylesheets comma separated as the first and only parameter.

Now it is easy: Use the global variable $editor_styles (here are your theme’s and parent theme’s editor stylesheets stored already), add the time of the file’s last modification as a parameter and rebuild the string.

As a plugin:

<?php # -*- coding: utf-8 -*-
/**
 * Plugin Name: Refresh Editor Stylesheets
 * Description: Enforces fresh editor stylesheets per version parameter.
 * Version:     2012.07.21
 * Author:      Fuxia
 * Plugin URI:  http://wordpress.stackexchange.com/q/33318/73
 * Author URI:  https://fuxia.me
 * License:     MIT
 * License URI: http://www.opensource.org/licenses/mit-license.php
 */

    add_filter( 'mce_css', 't5_fresh_editor_style' );

    /**
     * Adds a parameter of the last modified time to all editor stylesheets.
     *
     * @wp-hook mce_css
     * @param  string $css Comma separated stylesheet URIs
     * @return string
     */
    function t5_fresh_editor_style( $css )
    {
        global $editor_styles;

        if ( empty ( $css ) or empty ( $editor_styles ) )
        {
            return $css;
        }

        // Modified copy of _WP_Editors::editor_settings()
        $mce_css   = array();
        $style_uri = get_stylesheet_directory_uri();
        $style_dir = get_stylesheet_directory();

        if ( is_child_theme() )
        {
            $template_uri = get_template_directory_uri();
            $template_dir = get_template_directory();

            foreach ( $editor_styles as $key => $file )
            {
                if ( $file && file_exists( "$template_dir/$file" ) )
                {
                    $mce_css[] = add_query_arg(
                        'version',
                        filemtime( "$template_dir/$file" ),
                        "$template_uri/$file"
                    );
                }
            }
        }

        foreach ( $editor_styles as $file )
        {
            if ( $file && file_exists( "$style_dir/$file" ) )
            {
                $mce_css[] = add_query_arg(
                    'version',
                    filemtime( "$style_dir/$file" ),
                    "$style_uri/$file"
                );
            }
        }

        return implode( ',', $mce_css );
    }

Instead of just calling add_editor_style with your CSS file, add a cache buster query string parameter:

add_action('admin_enqueue_scripts', function(){
  if(is_admin()){
    add_editor_style(get_template_directory_uri().'/assets/css/editor.css?1');
  }
});

The problem with accepted answer in latest versions I assume is $editor_styles array only contains stylesheets added using theme, so as a result it strips rest of the stylesheets added by core wordpress or plugins from the returning string.

Following is the solution I came up with after tweaking the code, you can use it in your functions.php file. My solution uses nested loop and checks for stylesheets present in $editor_styles array, and appends the last modified time as parameter to query string and updates the value in array.

add_filter('mce_css', 'fresh_editor_style');

function fresh_editor_style($mce_css_string){
    global $editor_styles;
    $mce_css_list = explode(',', $mce_css_string);

    foreach ($editor_styles as $filename){
        foreach($mce_css_list as $key => $fileurl){
            if(strstr($fileurl, '/' . $filename)){
                $filetime = filemtime(get_stylesheet_directory() . '/' . $filename);
                $mce_css_list[$key] =  add_query_arg('time', $filetime, $fileurl);
            }
        }
    }

    return implode(',', $mce_css_list);
}

I couldn't get toscho's answer to work for the current version of WordPress (4.7.2), and that seems to be because the TinyMCE init array has a cache_suffix set to 'wp-mce-' . $tinymce_version.

So instead, you can just overwrite that with the tiny_mce_before_init filter, like so:

function wpse33318_tiny_mce_before_init( $mce_init ) {

    $mce_init['cache_suffix'] = 'v=123';

    return $mce_init;    
}
add_filter( 'tiny_mce_before_init', 'wpse33318_tiny_mce_before_init' );

Of course, this isn't nearly as good as filemtime(), but at least this works in 4.7.2.

Note: This also adds the cache buster to other editor styles (like skin.min.css, content.min.css, dashicons.min.css, and wp-content.css)

I had the same issue (2012, WP 3.4.2 !!). Possible solutions while this bug is around:

1) If you use firebug, [x]Disable Browser Cache in the Net panel helps. I even had a very strange issue, that the cached editor-style briefly appears (in a css-filtered) Firebug net panel for a split second, than disappears again. Took screenshots to prove to myself.

2) A full browser cache clear helps. For whatever reason thereafter the issue did not reappear.

3) Lastly, my preferred advice, if you have to also make sure, i.e. your clients on staging or live server get your incremental improvements (w/o any annoying cache clearance advice):

Relocate the file and keep counting up:

// add_editor_style('editor-style-01.css'); bump for every deployment
// add_editor_style('editor-style-02.css');
add_editor_style('editor-style-03.css');

Hacky, but reliable.