"A single solid block probably wouldn't be much good for anything other than 1:1 scale models though."
Well, you *could* print doors with hinges in place using a small amount of support material that you cut off once the print is finished. However, you couldn't do that for internal combustion engine parts because of the part tolerances needed. (Plus I defy anyone to print a cylinder ring in place with the spring it needs to actually function properly....)
As an aside, I seem to remember a story about an abandoned alien subway / tube train which the human archeologists thought had been cast in one piece, which is possible for some simple 3D designs. We're not there yet, but we're getting closer. The story might have been called "The subway of Kazoo" or something similar...