I think Diplomacy also requires you to have Entrenchment.

They are supposively going to release the games at once into retail stores when all of the mini-x-packs were done, not sure if that is still in the mix

Yeah I know that's what Blair said (think it was blair, it was definitely someone tho). There will be a box set of normal sins, entrenchment, and diplomacy all together the same time when diplomancy is released by itself (online only tho). So that probably means the only way you can get a box of diplo is to buy the game all over again.