[joechung]

The problem with implementation as design is that the implementation can be imperfect (underspecified, overspecified, or just plain broken) and thus can result in a difference between intent ("what you wanted to say") to reality ("what you actually said").

Then you get into the curious scenario where people trying to implement something whose design document is an implementation (e.g., the Ruby programming language, IE 7 "compatibility" mode) have to reproduce everything in the implementation, even the bugs, to be consistent with the design.