parent vs closest
$('.button_30').click(function(){
    $(this).closest('.portlet').find('.portlet_content').text("foo");
});​

$('.button_30').click(function(){
    $(this).parent('.portlet').find('.portlet_content').text("foo");
});​

<div class="portlet portlet_30">

    <div class="portlet_header portlet_header_30">
        header content here
    </div>

    <div class="portlet_sub_header portlet_sub_header_30">
        <input type="text" class="textbox_30" />
    </div>

    <div class="portlet_content portlet_content_30">
        results go here
    </div>

    <div class="portlet_footer portlet_footer_30">
        <input type="button" class="button_30" />
    </div>

</div>

<div class="portlet portlet_30">

    <div class="portlet_header portlet_header_30">
        header content here
    </div>

    <div class="portlet_sub_header portlet_sub_header_30">
        <input type="text" class="textbox_30 />
    </div>

    <div class="portlet_content portlet_content_30">
        results go here
    </div>

    <div class="portlet_footer portlet_footer_30">
        <input type="button" class="button_30" />
    </div>

</div>

$('.button_30').click(function(){
    $(this).parents('.portlet').find('.portlet_content').text("foo");
});​

$('.button_30').click(function(){
    $(this).parents('.portlet:eq(0)').find('.portlet_content').text("foo");
});​