Cacher is the code snippet organizer for pro developers

/**
 * Filter callback to add image sizes to Media Uploader
 *
 * WP 3.3 beta adds a new filter 'image_size_names_choose' to
 * the list of image sizes which are displayed in the Media Uploader
 * after an image has been uploaded.
 *
 * See image_size_input_fields() in wp-admin/includes/media.php
 *
 * Tested with WP 3.3 beta 1
 *
 * @uses get_intermediate_image_sizes()
 *
 * @param $sizes, array of default image sizes (associative array)
 * @return $new_sizes, array of all image sizes (associative array)
 * @author Ade Walker http://www.studiograsshopper.ch
 */
function sgr_display_image_size_names_muploader( $sizes ) {

    $new_sizes = array();

    $added_sizes = get_intermediate_image_sizes();

    // $added_sizes is an indexed array, therefore need to convert it
    // to associative array, using $value for $key and $value
    foreach( $added_sizes as $key => $value) {
        $new_sizes[$value] = $value;
    }

    // This preserves the labels in $sizes, and merges the two arrays
    $new_sizes = array_merge( $new_sizes, $sizes );

    return $new_sizes;
}
add_filter('image_size_names_choose', 'sgr_display_image_size_names_muploader', 11, 1);