(untested)

 <?php
 function widget_list_newest_blogs($args) {
        extract($args);

        if(function_exists(fimii_wpmu_newest_blogs)) {
                 echo $before_widget;

                 echo $before_title . 'Recently Created Blogs' . $after_title;
                        echo "<ul>\n";
                        fimii_wpmu_newest_blogs(5);
                        echo "</ul>\n";
                 echo $after_widget;
        } else {
                echo "Error - function list_all_wpmu_blogs not found";
        }
 }
 register_sidebar_widget('List Newest Blogs', 'widget_list_newest_blogs');
 ?>