This is because the_title() echos the post title (see the linked documentation). Use get_the_title() instead which returns the title as a string.

Edit

You have two options:

1. Use get_the_title() to return, rather than echo, the post title
2. Filter the_title to echo a custom string as the post title

Using get_the_title()

<?php
// NOTE: Inside the Loop,
// or else pass $post->ID
// as a parameter
$post_title = get_the_title();
?>

Using the_title filter

<?php
function wpse48523_filter_the_title( $title ) {
    // Modify or replace $title
    // then return the result
    // For example, to replace,
    // simply return your own value:
    // return 'SOME CUSTOM STRING';
    //
    // Or, you can append the original title
    // return $title . 'SOME CUSTOM STRING'
    //
    // Just be sure to return *something*
    return $title . ' appended string';
}
add_filter( 'the_title', 'wpse48523_filter_the_title' );
?>