From http://www.wprecipes.com/how-to-change-author-url-base-on-your-wordpress-site

change_author_base.php

add_action('init', 'cng_author_base');
function cng_author_base() {
    global $wp_rewrite;
    $author_slug = 'profile'; // change slug name
    $wp_rewrite->author_base = $author_slug;
}