form.html.haml

%h1 Terms of Service
%p You must agree to the TOS in order to submit this form.
%p
  %input{:type => "checkbox", :id => "agree_to_tos"}
%p
  = submit_tag "Process Payment", :id => "submit_if_agree_to_tos"

site.js

// Disable the submit button if the TOS checkbox is un-checked
var checkbox = $('.check_box input#agree_to_tos');
var submit = $('input#submit_if_agree_to_tos');

submit.attr('disabled', 'disabled');
checkbox.click(function() {
	if( $(this).attr("checked") ) {
		submit.removeAttr('disabled');
	} else {
		submit.attr('disabled', 'disabled');
	}
});