It's always been vertical for me

Horizontal, it doesn't fit right vertically in my entertainment center.

Horizontal, because when spinning a disk in a vertical position, you have:
F(side moving downward) = F(motor) + F(gravity)
F(side moving upward) = F(motor) - F(gravity)

Now, F(Gravity) is the same for both sides, and F(Motor) must be the same due to the fact that they are being spun by the same motor.

This means that the difference of the forces != 0, which means there are mismatched forces while spinning which can lead to damaged disks. Now, I assume Nintendo has accounted for this, but even so - better safe than sorry.

EDIT: I realize that the difference in forces is minute, and my math is very simplified; I just prefer to play it safe, and don't wanna type out the actual math.  I have done it in a more thorough fashion though

i keep it vertical

Horizontal, so it will fit.

Horizontal when it was in the TV cabinet, but then i got a PS3 and it would not fit in the cabinet, and i could not have the PS3 out sitting on top all alone, so i moved the wii up next to it (vertically) and there they live happy together.

lol opps forgot to invert the pic, all i can say is "to the left, to the left" oh god, awful song

Vertical, right of my vertical PS3

Horizontal. Too much of an old school gamer. The only time we had a console vertical was the PS1 when it wouldn't read disks

Vertical. Right next to my television like Shigeru Miyamoto wanted!

 WoW, how do you get the controler to float like that