Twenty Eleven uses a pluggable callback function to show the comments, so you can create a copy of it in functions.php of your child theme, and edit it;
the function code starts with:
function twentyeleven_comment( $comment, $args, $depth ) {
you will need to copy the full function into functions.php of the child theme.
you could then rewrite this section of the code:
/* translators: 1: comment author, 2: date and time */
printf( __( '%1$s on %2$s <span class="says">said:</span>', 'twentyeleven' ),
sprintf( '<span class="fn">%s</span>', get_comment_author_link() ),
sprintf( '<a href="%1$s"><time datetime="%2$s">%3$s</time></a>',
esc_url( get_comment_link( $comment->comment_ID ) ),
get_comment_time( 'c' ),
/* translators: 1: date, 2: time */
sprintf( __( '%1$s at %2$s', 'twentyeleven' ), get_comment_date(), get_comment_time() )
)
);
for example with:
// shorten last name of comment author to one initial //
$url = get_comment_author_url( $comment_ID );
$author = get_comment_author( $comment_ID );
$author_parts = explode( ' ', $author );
if( count( $author_parts ) >1 ) {
$author_last = array_pop( $author_parts );
$author_last = substr( $author_last, 0, 1) . '.';
array_push( $author_parts, $author_last );
$author = implode( ' ', $author_parts );
}
if ( empty( $url ) || 'http://' == $url )
$comment_author_link = $author;
else
$comment_author_link = "<a href='$url' rel='external nofollow' class='url'>$author</a>";
/* translators: 1: comment author, 2: date and time */
printf( __( '%1$s on %2$s <span class="says">said:</span>', 'twentyeleven' ),
sprintf( '<span class="fn">%s</span>', $comment_author_link ),
sprintf( '<a href="%1$s"><time datetime="%2$s">%3$s</time></a>',
esc_url( get_comment_link( $comment->comment_ID ) ),
get_comment_time( 'c' ),
/* translators: 1: date, 2: time */
sprintf( __( '%1$s at %2$s', 'twentyeleven' ), get_comment_date(), get_comment_time() )
)
);
http://codex.wordpress.org/Template_Tags/get_comment_author_link
not widely tested