Yeah, I've been expecting someone to work up a system where:

- source file is .md

- file is compiled to .pdf _and_ the .md source file is included as an attachment

- when working with the file beyond viewing as a .pdf the .md is extracted and used instead of the .pdf

The LaTeX folks have a similar system ages ago where the .tex source would be included in a .pdf made from a .tex file for embedding in documents so that it could be sent in say an e-mail and then edited by the recipient --- absolutely awesome for discussing math via e-mail.