Yeah...I found the solution last week exactly with the contentService.

This is an example code:

using System;
using Umbraco.Core;
using Umbraco.Core.Events;
using Umbraco.Core.Models;
using Umbraco.Core.Publishing;
using Umbraco.Core.Services;

namespace myNameSpace

{
    public class myHandler : ApplicationEventHandler
    {

        protected override void ApplicationStarted(UmbracoApplicationBase umbracoApplication, ApplicationContext applicationContext)
        {
            ContentService.Saving += ContentServiceSaving;
        }

        private void ContentServiceSaving(IContentService sender, SaveEventArgs<IContent> args)
        {
            foreach (var node in args.SavedEntities)
            {
                foreach (var property in node.Properties)
                {
                    if ((property.Alias == "title") && (String.IsNullOrEmpty(property.Value.ToString()))) 
                    { 
                        property.Value = node.Name; 
                    }

                    if ((property.Alias == "date") && (String.IsNullOrEmpty(property.Value.ToString())))
                    {
                        property.Value = DateTime.Now;
                    }
                }
            }
        }
    }
}

That's all!!!

Glad to help.

Adriano