wp-active-plugins-data.php

<?php
/*
Plugin Name: WP Active Plugins Data
Plugin URI: http://mikeschinkel.com/wordpress-plugins/wp-active-plugins-data/
Description: Loads Plugin Data on Plugin Activation and Persists to wp_options for quick retrieval.
Version: 0.1
Author: Mike Schinkel
Author URI: http://mikeschinkel.com
Note: Written for http://wordpress.stackexchange.com/questions/361/is-there-a-way-for-a-plug-in-to-get-its-own-version-number
*/

require_once(ABSPATH.'wp-admin/includes/plugin.php');

function get_active_plugin_version($plugin_path_file, $sitewide = false) {
	return get_active_plugin_attribute($plugin_path_file,'Version');
}
function get_active_plugin_attribute($plugin_path_file, $attribute) {
	$all_plugins_data = get_active_plugins_data($plugin_path_file,$sitewide);
	return (isset($all_plugins_data[$attribute]) ? $all_plugins_data[$attribute] : false);
}

function get_active_plugins_data($plugin_path_file, $sitewide = false) {
	$failsafe = false;
	$plugin = plugin_basename(trim($plugin_path_file));
	$sitewide = (is_multisite() && ( $sitewide || is_network_only_plugin($plugin)));
	if ($sitewide) {
		$all_plugins_data = get_site_option('active_sitewide_plugin_data',array());
	} else {
		$all_plugins_data = get_option('active_plugin_data',array());
	}
	if (!$failsafe && !is_array($all_plugins_data) || count($all_plugins_data)==0) {
		$failsafe = true; // Don't risk infinite recursion
		if ($sitewide) {
			$active_plugins = get_site_option('active_sitewide_plugins',array());
		} else {
			$active_plugins = get_option('active_plugins',array());
		}
		persist_active_plugin_data(null,$active_plugins,$sitewide);
		$all_plugins_data = get_active_plugin_version($plugin_path_file,$sitewide);
	}
	return $all_plugins_data[$plugin_path_file];
}
add_action('update_site_option_active_sitewide_plugins','persist_sitewide_active_plugin_data',10,2);
function persist_sitewide_active_plugin_data($option, $plugins) {
	persist_active_plugin_data(null,$plugins,'sitewide');
}
add_filter('update_option_active_plugins','persist_active_plugin_data',10,2);
function persist_active_plugin_data($old_plugins, $new_plugins, $sitewide=false) {
	$active_plugin_data = array_flip($new_plugins);
	$plugin_dir = WP_PLUGIN_DIR;
	foreach($new_plugins as $plugin) {
		$active_plugin_data[$plugin] = get_plugin_data("$plugin_dir/$plugin");
	}
	if ($sitewide)
		update_site_option('active_sitewide_plugin_data',$active_plugin_data);
	else
		update_site_option('active_plugin_data',$active_plugin_data);
}