Make no assumption that this is the real deal but at least in one case, it is worth a gander.
German language site GameFront has put up two potential GameCube logos submitted to them by a reader apparently. Of course nobody should assume these are the real deal... nobody is probably going to say anything until the final annoucement next Wednesday... but still, this one particular logo concept is eye-catching.
Translated from the page here is what GameFront itself said on the matter:
From Naojuki from Japan these pictures of the Gamecube Logos achieved us - unfortunately still no acknowledgement of Nintendo is present whether this Logos is genuinly or officially or only designs. Therefore: Without guarantee!