Braid creator Jonathan Blow has revealed that he shipped a hidden level editor in the PC version, which came out last Friday.
"The way it works is you can make levels with the editor (up to a full game, potentially) and run that with -universe later... also a tool will be released that lets you take Photoshop files and import them into the game, if you want to put new graphics in your levels," he said in his original Steam forums response.
Apparently you can replace sounds, levels and graphics, although "doing two players isn't really going to work". The network code is in another castle!
Braid, just in case you missed all the Xbox 360 hullabaloo, is a 2D platform puzzle game where you manipulate time while basically having an existential crisis. It's very pretty and there's a dinosaur in it.