<script type="text/javascript">
checkobj = 0;

function checkAgree(obj) {
if (obj.checked) checkobj++;
else checkobj--;
if (checkobj <= 0) document.check_agree_form.elements['submit_button'].disabled = true;
else document.check_agree_form.elements['submit_button'].disabled = false;
}
</script>

<form action="#" method="post" name="check_agree_form">
<p>
<input name="agree" type="checkbox" value="agree" ><input disabled="" name="submit_button" type="submit" value="Отправить"/>
</p>
</form>

Демо