The administrator of the windowsmania.pl forum has posted the above video said to be of Windows phone 7.8 running on the Nokia Lumia 800.
DjTonka insists it is the real deal, and from the animation of the People Hub tile we assume he is correct.
He does not reveal any new features besides the new start screen, and will not be releasing the ROM just yet.
Read more in this WindowsMania.pl thread.
See some pictures after the break.
Thanks Ricky for the tip.