Change "save" to "submit"?

It is totally possible but it would require you editing a Drupal core php file, which is not recommended for non-advanced users.
Also to mention, if you ever upgraded core, eg (Drupal 7.0 -> Drupal 7.2) then you would have to reapply the changes, and they may be in a different spot.

http://drupal.org/project/stringoverrides

buttonalter.info

; $Id$
name = ButtonAlter
description = Button Text Override
core = 6.x
package = 000 Custom Modules

buttonalter.module

<?php

function buttonalter_form_alter(&$form, $form_state, $form_id) {
	if($form_id=="apt_rental_listing_node_form") {
		$form['buttons']['submit']['#value'] = t('Submit');
		unset ($form['buttons']['preview']);
	}
	if($form_id=="res_mgr_listing_node_form") {
		$form['buttons']['submit']['#value'] = t('Submit');
		unset ($form['buttons']['preview']);
	}
}

this code changes the "save" button to "submit" and suppresses the "preview" button for these two content types -- apt_rental_listing & res_mgr_listing.

Edit: this is a drupal 6.x solution that I have used before. It may not work with D7. But maybe it will give some direction regardless.

In Drupal 7 its:

$form['actions']['submit']['#value'] = t('Submit');

I've tried the custom module. No dice here. In looking at other posts online I see some differences.

Your suggestion...
function yourmodule_form_alter(&$form, $form_state, $form_id) {

Other suggestion...
function yourmodule_form_alter(&$form, &$form_state, $form_id) {

And as in the MailChimp Module I'm trying to override instead of just hacking their module..
function yourmodule_form_alter($form, &$form_state, $form_id) {

And, I've also tried this variation...

$form['submit'] = array(
	'#type' => 'submit',
	'#value' => t('Subscribe'),
);

When this one didn't work...
$form['actions']['submit']['#value'] = t('Subscribe');

String overrides is not a solution because it will change everything from "Save" to "Subscribe" which would not make sense for node publishing where "Submit" is preferred.