Tested and got the extension working with Rowan's textbox field.

Added the Preview link back to textareas without a formatter (just
returns the raw HTML anyway).

assets/preview_textarea.publish.js

@@ -1,27 +1,14 @@
 /*
-
+ * Preview Textarea
+ * 
+ * Grab value of a textarea and run it through that textarea's formatter,
+ * then display a live preview of the result.
  */
 
 jQuery(document).ready(function($) {
-	/*
-	 * Add a "Preview" link next to each textarea, as well as divs we'll be using to store stuff
-	 */
-	var ownerDocument = top.document;
-	$('body')
-		.append(
-			$(document.createElement('div')).attr('id','preview-textarea')
-				.append(
-					$(document.createElement('div')).attr('id','preview-textarea-title')
-				).append(
-					$(document.createElement('div')).attr('id','preview-textarea-content')
-				)
-		).append(
-			$(document.createElement('div')).attr('id','preview-textarea-overlay')
-		);
-
-	$('textarea[class!=""]').before(
-		$(document.createElement('a')).text('Preview').addClass('preview-textarea-button')
-	);
+	// Add a "Preview" link next to each textarea, as well as divs we'll be using to store stuff
+	$('body').append('<div id="preview-textarea"><div id="preview-textarea-title"></div><div id="preview-textarea-content"></div></div><div id="preview-textarea-overlay"></div>');
+	$('textarea').before('<a class="preview-textarea-button">Preview</a>');
 
 	$('a.preview-textarea-button').click(function(event) {
 		event.preventDefault();
@@ -30,6 +17,7 @@ jQuery(document).ready(function($) {
 			$(this).after('&#160;<strong>This textarea is empty!</strong>').next().delay(5000).fadeOut('slow');
 		}
 		else {
+			// Let's drop some AJAX.
 			$.ajax({
 				type: 'POST',
 				dataType: 'html',
@@ -43,23 +31,25 @@ jQuery(document).ready(function($) {
 					var windowWidth = $(window.top).width();
 
 					var fieldLabel = thePackage.parent().clone().children().remove().end().text();
-
-					$('#preview-textarea-title', ownerDocument).html('Preview of ' + fieldLabel + ' using the ' + $(xml).find("formatter").text() + ' formatter <a title="Close preview">Close</a>');
+					var formatter_name = ($(xml).find("formatter").text() == 'None') ? 'no formatter' : 'the <em>' + $(xml).find("formatter").text() + '</em> formatter';
+					$('#preview-textarea-title', top.document).html('Preview of <em>' + fieldLabel + '</em> using ' + formatter_name + ' <a title="Close preview">Close</a>');
 
-					$('#preview-textarea-content', ownerDocument).html(($(xml).find("preview").html()));
-					var previewTop = windowHeight/2 - $('#preview-textarea', ownerDocument).height()/2;
+					// Magic to determine the width and height of the preview box and the overlay
+					$('#preview-textarea-content', top.document).html(($(xml).find("preview").html()));
+					var previewTop = windowHeight/2 - $('#preview-textarea', top.document).height()/2;
 					if(previewTop < 0) { previewTop = 100; }
-					$('#preview-textarea', ownerDocument).css({
+					$('#preview-textarea', top.document).css({
 						'top': previewTop,
-						'left': windowWidth/2 - $('#preview-textarea', ownerDocument).width()/2
+						'left': windowWidth/2 - $('#preview-textarea', top.document).width()/2
 						});
 
-					var overlayHeight = ($(ownerDocument).height() > $('#preview-textarea', ownerDocument).height()) ? $(ownerDocument).height() : $('#preview-textarea', ownerDocument).height() + 200;
+					var overlayHeight = ($(top.document).height() > $('#preview-textarea', top.document).height()) ? $(top.document).height() : $('#preview-textarea', top.document).height() + 200;
 
-					$('#preview-textarea-overlay', ownerDocument).width(windowWidth).height(overlayHeight).fadeTo('fast',0.75);
-					$('#preview-textarea', ownerDocument).fadeIn('fast');
-					$('#preview-textarea-title a, #preview-textarea-overlay', ownerDocument).click(function() {
-						$('#preview-textarea, #preview-textarea-overlay', ownerDocument).fadeOut();
+					// Close the preview if the overlay or the "Close" anchor are clicked
+					$('#preview-textarea-overlay', top.document).width(windowWidth).height(overlayHeight).fadeTo('fast',0.75);
+					$('#preview-textarea', top.document).fadeIn('fast');
+					$('#preview-textarea-title a, #preview-textarea-overlay', top.document).click(function() {
+						$('#preview-textarea, #preview-textarea-overlay', top.document).fadeOut();
 					});
 				},
 				cache: false

content/content.preview.php

@@ -6,18 +6,29 @@
 	class contentExtensionPreview_textareaPreview extends AjaxPage {
 
 		public function view() {
-
-
+			// Determine which formatter to use
+			// The textarea keeps this info in its class attribute, but sometimes there might be additional classes.
+			// We'll grab one that matches the installed formatters.
 			$fM = new TextformatterManager($this);
-			$formatter = $fM->create($_POST['formatter']);
-			$formatter_about = $formatter->about();
+			$formatter_handle = array_pop(array_intersect(array_keys($fM->listAll()), explode(' ', $_POST['formatter'])));
 
+			// We pass the full formatter name back for use in the preview display
 			$format_name = new XMLElement('formatter');
-			$format_name->setValue($formatter_about['name']);
-			$this->_Result->appendChild($format_name);
-
 			$preview = new XMLElement('preview');
-			$preview->setValue($formatter->run($_POST['formatText']));
+
+			if(empty($formatter_handle)) {
+				$format_name->setValue('None');
+				$preview->setValue($_POST['formatText']);
+			}
+			else {
+				$formatter = $fM->create($formatter_handle);
+				$formatter_about = $formatter->about();
+
+				$format_name->setValue($formatter_about['name']);
+				$preview->setValue($formatter->run($_POST['formatText']));
+			}
+
+			$this->_Result->appendChild($format_name);
 			$this->_Result->appendChild($preview);
 		}
 

extension.driver.php

@@ -5,8 +5,8 @@
 		public function about() {
 			return array(
 				'name' => 'Preview Textarea',
-				'version' => '0.1',
-				'release-date' => '',
+				'version' => '1.0',
+				'release-date' => '2011-08-16',
 				'author' => array(
 					'name' => 'Ben Babcock',
 					'website' => 'http://tachyondecay.net/',