**UPDATE**
MSN has been replaced by Bing in the latest version:

New in Version 3.1.3 (2009-06-07):

* Changed MSN Live Search to Bing
* Exclude categories also now exludes the category itself and not only the posts
* Pings now use the new WordPress HTTP API instead of Snoopy
* Fixed bug that in localized WP installations priorities could not be saved
* The sitemap cron job is now cleared after a manual rebuild or after changing the config
* Adjusted style of admin area for WP 2.8 and refreshed icons
* Disabled the “Exclude categories” feature for WP 2.5.1, since it doesn’t have the required functions yet

Like many WP users, I have Google XML Sitemaps installed to automatically generate new sitemaps and ping major search engines after every new post. I wanted to add Bing to the list so i dug around the code a bit and figured out that Bing’s sitemap ping service is almost exactly the same as MSN’s. It doesn’t take much to get Bing added to the list of search engines:

sitemap-core.php:382

	var $_usedBing = false;
	var $_bingUrl = '';
	var $_bingSuccess = false;
	var $_bingStartTime = 0;
	var $_bingEndTime = 0;
 
	function StartBingPing($url) {
		$this->_usedBing = true;
		$this->_bingUrl = $url;
		$this->_bingStartTime = $this->GetMicrotimeFloat();
 
		$this->Save();
	}
 
	function EndBingPing($success) {
		$this->_bingEndTime = $this->GetMicrotimeFloat();
		$this->_bingSuccess = $success;
 
		$this->Save();
	}
 
	function GetBingTime() {
		return round($this->_bingEndTime - $this->_bingStartTime,2);
	}

sitemap-core.php:1047

		$this->_options["sm_b_pingbing"]=true;				//Auto ping Bing

sitemap-core.php:2254

		//Ping Bing
		if($this->GetOption("b_pingbing") && !empty($pingUrl)) {
			$sPingUrl="http://www.bing.com/webmaster/ping.aspx?siteMap=" . urlencode($pingUrl);
			$status->StartBingPing($sPingUrl);
			$pingres=$this->RemoteOpen($sPingUrl);
 
			if($pingres==NULL || $pingres===false || strpos($pingres,"Thanks for submitting your sitemap")===false) {
				$status->EndBingPing(false,$this->_lastError);
			} else {
				$status->EndBingPing(true);
			}
		}

sitemap-ui.php:650

									if($status->_usedBing) {
										if($status->_bingSuccess) {
											echo "<li>" .__("BING was <b>successfully notified</b> about changes.",'sitemap'). "</li>";
											$at = $status->GetBingTime();
											if($at>4) {
												echo "<li class=\sm_optimize\">" . str_replace("%time%",$at,__("It took %time% seconds to notify BING.com, maybe you want to disable this feature to reduce the building time.",'sitemap')) . "</li>";
											}
										} else {
											echo "<li class=\"sm_error\">" . str_replace("%s",$status->_bingUrl,__('There was a problem while notifying BING.com. View result','sitemap')) . "</li>";
										}
									}

sitemap-ui.php:767

							<li>
								<input type="checkbox" id="sm_b_pingbing" name="sm_b_pingbing" <?php echo ($this-/>sg->GetOption("b_pingbing")==true?"checked=\"checked\"":"") ?> />
								<label for="sm_b_pingbing">< ?php _e('Notify BING about updates of your Blog', 'sitemap') ?></label><br />
								<small>< ?php echo str_replace("%s",$this->sg->GetRedirectLink('sitemap-lwt'),__('No registration required, but you can join the BING Webmaster Tools to check crawling statistics.','sitemap')); ?></small>
							</li>

After you’ve modified the files, check go to your admin section > Settings > XML-Sitemap and regenerate your sitemap manually. You should see “BING was successfully notified about changes.” right under the MSN entry. Way to go!