the checkbox code should be:
<input <?php if (!(strcmp((ValidatedField("SendInNameV3_156","OK to join miling list"))) {echo "checked=\"checked\"";} ?> name="join_mail_list" type="checkbox" id="join_mail_list" value="OK to join miling list" />
you have a hard coded " checked="checked"" at the end that is overriding the dynamic setting.