I've been looking for the King Kong demo too. I know my market place has updated since I have new content like the Outfit demo, but I have never seen a King Kong Demo. And now it looks like I can't see the Ridge Racer demo either.
I have two accounts, one in the US that was transfered from my Original Xbox and a new one (silver, that has been gold for like two months now) I made to see the Japan content. Niehter show those two demos. I wonder what else I'm missing, and why. My xbox was a launch 360 box and has all the updates. Also I've never had a problem with it untill this.
I found that setting the parental lock would block certain content so I disabled all of that. This enabled content like the condemded demo and others but sadly not the King Kong or Ridge Racer demos. I wonder what else I'll miss in the future. I think the downloadable demos are great and a big selling point for me. I think I'm missing the latest two Live Arcade games too.
I really hope someone figures this out soon, before I miss something good and have to rent the game.
My thoughts are it is xbox specific since niether of my accounts show the content.
HaloGhost0