creating a $_SESSION var w/ AJAX, and jQuery, but is disappearing on subsequent pages

Yesterday i coded a form checkbox to set a PHP Session variable on tpl_shopping_cart_default.php. i used jQuery and Ajax to do so. below is the code.

the problem is that the variable $_SESSION['hm-agreement'], is disappearing on subsequent pages, like for ex: tpl_shopping_cart_default, or log-in etc. i do a var_dump($_SESSION) to check this.

in the code below i've even echoed out the the SESSION var in a callback function via a js alert and it is echoing back correctly.

$(document).ready(function(){
	$('input:checkbox').click(function(){
		if(this.checked) {
			state = 'checked';
		} else {
			state = 'unchecked';
		}
		$.get('pyro-agreement.php', {action:state}, function(data) {alert(data); })
	}); })

am hesitant to supply a link to the dev. site here but could do so privately.
i did a little study to test the code in a simpler environment and seems to be working as expected.

http://tomcarden.net.previewdns.com/php-ajax-session-var-study/intro.php

any help on this would be appreciated. i don't understand why the variable once created doesn't exist on sub-sequent pages. i've

If your ajax script hasn't started the same session that your live page is using, then none of its variables will carry over.

Simpler would be to have your javascript set the HTML for the corresponding input field on your actual page, and then have that normal page (nothing to do with ajax) submit that field with all the rest of its submission data, and parse it just like all the other input fields on the page, and have it set the session variable from there.
example: the checkout-comments field or the checkout terms-and-conditions field.