Re: the DC barrel at the back of the Arduino - after much frustration with the poor quality of the Mega 2560 (the layout is dimensional way off and a lot of the connections are missing) - I have reworked it. Included in that are connections for the barrel jack. Once I have fully road tested the new version I will post it here for feedback and then GIT it. If the Mega meets approval, Iwill do the same for the Uno and a couple of the other other popular 'duino boards. It is quite a radical departure especially in the schematic view, so people will probably hate it.
I was going to create a plug for it. Looks like you beat me to it @Vanepp, big thanks. Even if I have to polish / tweak it a bit, it's a big help.