<?php $search_text = "Search"; ?>
<form method="get" id="searchform"
action="<?php bloginfo('home'); ?>/">
<input type="text" value="<?php echo $search_text; ?>"
name="s" id="s"
onblur="if (this.value == '')
{this.value = '<?php echo $search_text; ?>';}"
onfocus="if (this.value == '<?php echo $search_text; ?>')
{this.value = '';}" />
<input type="hidden" id="searchsubmit" />
</form>
Reference : http://www.dagondesign.com/articles/wordpress-search-form-without-search-button/