Ok, updated my site and it seems to have gone ok...
Some questions...
1. What is _mediaidx.disp.php ?
2. Why has .disp been added to all these things like arcdir etc etc.
3. What's changed that a plugin that worked in the previous beta now causes a major error on the plugins Admin page.
The plugin is _acronym.plugin.php

<?php
/**
 * This file implements the Beano's Acronym plugin for b2evolution
 *
 * b2evolution - {@link http://b2evolution.net/}
 * Released under GNU GPL License - {@link http://b2evolution.net/about/license.html}
 *
 * Beano's Acronym Plugin (v0.1 alpha - 05/09/2006)
 * copyright  (c)2006 by Steve Ferson (http://www.steveferson.com)
 *
 */
if( !defined('EVO_MAIN_INIT') ) die( 'Please, do not access this page directly.' );


class acronym_plugin extends Plugin {
	var $code = 'b2acro';
	var $name = 'Acronyms';
	var $priority = 80;
	var $version = '1.8';
	var $apply_rendering = 'opt-out';

	/**
	 * Acronym search array (regex)
	 */
	var $search;

	/**
	 * Replace array
	 */
	var $replace;

	/**
	 * Acronym definitions
	 */
	var $acronyms;

	/**
	 * Acronym titles / explanations
	 */
	var $titles;

	/**
	 * Constructor
	 */
	function acronym_plugin() {
		$this->short_desc = T_('Acronym Explanations');
		$this->long_desc = T_('This renderer will convert groups of recognised initials (acronyms) in a post into HTML &lt;acronym&gt; elements with appropriate titles.');

		/**
		 * Acronym configuration.
		 * TODO: Store acronym/title pairs in database table
		 */
		require dirname(__FILE__).'/acronym_plugin/_conf.php';
	}


	/**
	 * Perform rendering
	 *
	 * @see Plugin::RenderItemAsHtml()
	 */
	function RenderItemAsHtml( & $params ) {
		if( ! isset( $this->search ) )	{	// We haven't prepared the acronyms yet
			$this->search = array();

			$tempacros = $this->acronyms;
			uksort($tempacros, array(&$this, 'orderbylength'));

			foreach($tempacros as $acronym => $desc) {
				$this->search[] = '/(\b)'. $acronym .'(\b)/';
				$this->replace[] = '$1<acronym title="'.$desc.'">'. $acronym .'</acronym>$2';
			}
		}


		// REPLACE:  But only in PCDATA, exclude text inside <code> and <pre> elements
		$content = & $params['data'];

		// Check if data contains <code> or <pre> elements
		if( stristr( $content, '<code' ) !== false || stristr( $content, '<pre' ) !== false )	{ 
			// Call ReplaceTagSafe() on everything outside <pre></pre> and <code></code>:
			$content = callback_on_non_matching_blocks( $content,
					'~<(code|pre)[^>]*>.*?</\1>~is',
					array( & $this, 'ReplaceTagSafe' ) );
		} else { // If there are no <code> or <pre> elements, it's safe to replace on the whole block of text
			$content = $this->ReplaceTagSafe($content);
		}

		return true;
	}


	/**
	 * This callback gets called once after every tags+text chunk
	 * @return string Text with replaced acronyms
	 */
	function preg_insert_acronyms_callback( $text )	{
		return preg_replace( $this->search, $this->replace, $text );
	}


	/**
	 * Replace acronyms outside HTML-tags in the text.
	 * @uses callback_on_non_matching_blocks()
	 */
	function ReplaceTagSafe($text) 	{
		return callback_on_non_matching_blocks( $text, '~<[^>]*>~', array(&$this, 'preg_insert_acronyms_callback') );
	}


	/**
	 * sorts the acronyms' array by length
	 * which allows greedy matching
	 * (ie will not match a 3 letter acronym if it's part of a 4 letter one)
	 */
	function orderbylength($a, $b) 	{
		if(($diff = strlen($b) - strlen($a)) == 0) {
			return strcmp($a, $b);
		}
		return $diff;
	}
}
?>

Cheers