http://blog.wolfire.com/2009/01/map-editor-video/
Seems really easy to use, and like a great way to preview stuff (also because it auto-updates textures, like max does)
Pre-ordering gives you access to the alpha (it's an alpha, don't expect much from it) of the game and the level editor.
edit: peek, right? Well, I don't think I can still change the title.