SKJmin has released his hack (via Joystiq), which includes all 8 worlds of Super Mario Bros. 3 and also includes 3 worlds of his own creation. The download includes "everything you need for patching... besides the ROM, because it's illegal!" The warnings continue: "Please remember, don't download this if you have not the original game from Nintendo!"
The levels have been changed, but the overworld, menus, and some other features of New Super Mario Bros. DS remain intact. Such is the nature of ROM hacks.
The trailer below shows off a lot of what you can expect.