is_home does not function

WP version: 1.5.3-beta 1

Problem:
the is_home() function always return TRUE either at main page or previous page(e.g: domain.com/blog/page/2/ )

Sample Code on index.php:
<div id="content" class="narrowcolumn">
<?php if (is_home()) { ?>
this is home.
<?php } ?>
<?php if (have_posts()) : ?>

Expected result:
Show “this is home” on main page, but does not show on previous pages.

Result getting:
It shows “this is home” on main page AND other pages.

Any idea? Thanks.