[Carpesimia]

Ok people, I got it working, and nicely. Here is the updated XML that I used to get it to work. *note* this requires 1 template EDIT in order to do it the way i did it.

<?xml version="1.0" encoding="ISO-8859-1"?>

<product productid="pm_replyall" active="1">
	<title>Reply to All - PM</title>
	<description>User can reply all when replying to a PM sent to multiple users</description>
	<version>4.0</version>
	<url>http://www.forumsmods.com/</url>
	<versioncheckurl />
	<dependencies>
		<dependency dependencytype="vbulletin" minversion="4.0.0 Alpha 1" maxversion="" />
	</dependencies>
	<codes>
	</codes>
	<templates>
		<template name="pm_replyall" templatetype="template" date="1292609646" username="vB Style" version="4.0"><![CDATA[ - <b>[<a href="private.php?{vb:raw session.sessionurl}do=newpm&amp;pmid={vb:raw pm.pmid}&amp;replyto=all" class="username">{vb:rawphrase pm_reply_to_all}</a>]</b>]]></template>
	</templates>
	<stylevardfns>
	</stylevardfns>
	<stylevars>
	</stylevars>
	<plugins>
		<plugin active="1" executionorder="5">
			<title>Cache Templates</title>
			<hookname>cache_templates</hookname>
			<phpcode><![CDATA[if (THIS_SCRIPT == 'private' AND $_REQUEST['do'] == 'showpm')
{
	$cache[] = 'pm_replyall';
}]]></phpcode>
		</plugin>
		<plugin active="1" executionorder="5">
			<title>Reply to All - PM</title>
			<hookname>private_complete</hookname>
			<phpcode><![CDATA[if ($vbulletin->options['pmreplyall'] AND ($_REQUEST['do'] == 'showpm' AND !empty($ccrecipients)))
{
	$templater = vB_Template::create('pm_replyall');
	$templater->register('pm', $pm);
	$repall = $templater->render();
	$page_templater->register('repall', $repall);
}
if ($vbulletin->options['pmreplyall'] AND ($_REQUEST['replyto'] == 'all' AND $_REQUEST['do'] == 'newpm' AND !empty($pm['touserarray'])))
{
	$touser = unserialize($pm['touserarray']);
	if (is_array($touser))
	{
		unset($touser['cc'][array_search($vbulletin->userinfo['username'], $touser['cc'])]);
		$pm['recipients'] .= implode(' ; ', $touser['cc']);
		$page_templater->register('pm', $pm);
	}
}]]></phpcode>
		</plugin>
	</plugins>
	<phrases>
		<phrasetype name="Private Messaging" fieldname="pm">
			<phrase name="pm_reply_to_all" date="1292609222" username="vB Style" version="4.0"><![CDATA[Reply to All]]></phrase>
		</phrasetype>
		<phrasetype name="vBulletin Settings" fieldname="vbsettings">
			<phrase name="setting_pmreplyall_desc" date="1292609743" username="vB Style" version="4.0"><![CDATA[Enable "Reply To All" when replying to a PM sent to multiple users]]></phrase>
			<phrase name="setting_pmreplyall_title" date="1292609743" username="vB Style" version="4.0"><![CDATA[Enable "Reply To All"]]></phrase>
		</phrasetype>
	</phrases>
	<options>
		<settinggroup name="pm" displayorder="440">
			<setting varname="pmreplyall" displayorder="150">
				<datatype>boolean</datatype>
				<optioncode>yesno</optioncode>
				<defaultvalue>1</defaultvalue>
			</setting>
		</settinggroup>
	</options>
	<helptopics>
	</helptopics>
	<cronentries>
	</cronentries>
	<faqentries>
	</faqentries>
</product>

Then, inside the pm_showpm template (in the Private Message Templates), add {vb:raw repall} wherever you want the REPLY ALL link to show. For example, here is where I put mine:

<div id="showpm">
	<vb:if condition="$show['recipients']">     
		<div class="abovepm floatcontainer">
			<vb:if condition="$ccrecipients">
				<div>
					<span class="bold">{vb:rawphrase recipients}: </span>
					<ol class="commalist">
						<vb:each from="ccrecipients" value="row">
							<li><a href="{vb:link member|js, {vb:raw row}}" class="username understate">{vb:raw row.username}</a>{vb:raw row.comma}</li>
						</vb:each> {vb:raw repall}
					</ol>
				</div>
			</vb:if>

I put mine right on the same line as each name. So the outcome looks like this:

Recipient users: user1, user2, user3 - [Reply to all]

That's it. I opted for the template edit because it let me put the reply all exactly where i wanted it, without trying to piggyback it on another variable.

I hope this helps...