update

update.cs

private void updateServerStatus()
{

	foreach (ServerItem server in ServerItems)
	{
		var capturedServer = server;
		server.ServerImage = "/Images/appbar.questionmark.rest.png";
		string url = server.ServerAddress;
		if (url.StartsWith("http://") || url.StartsWith("https://")) { }
		else
		{
			url = "http://" + url;
		}
		try
		{
			HttpWebRequest req = (HttpWebRequest)WebRequest.Create(url);
			req.BeginGetResponse(result => updateServerStatusImage(result, capturedServer, (HttpWebResponse)req.EndGetResponse(result)), null);
		}
		catch
		{
			server.ServerImage = "/Images/appbar.cancel.rest.png";
			serverDB.SubmitChanges();
		}

	}
}


private void updateServerStatusImage(IAsyncResult result, ServerItem server, HttpWebResponse response)
{
    if (response.StatusCode == HttpStatusCode.OK)
    {
        Deployment.Current.Dispatcher.BeginInvoke(() => 
        {
            server.ServerImage = "/Images/circle-green.png";
            serverDB.SubmitChanges();
        });
    }
    else
    {
        Deployment.Current.Dispatcher.BeginInvoke(() =>
        {
            server.ServerImage = "/Images/circle-red.png";
            serverDB.SubmitChanges();
        });
    }
}

I've gotten rid of the endWebRequest method (cleaner to me :).
Also note that I've added a captureServer variable in the loop. What you had before was accessing a modified closure. This mean that your asynchronous code could have accessed a reference to a variable the likely be updated by the next iteration of the loop before the asynchronous code (the lambda) was executed.