Jon Blow’s confirmed on the Steam forum that a level editor has been included in the PC version of Braid:
Yeah, after I get a new version out in a few days that fixes the problems some people are having, and when more people have played/finished the game, I am going to post some documentation for the editor. 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.
The editor’s already in the game, apparently. Press F11 and see what happens.