Speaking to Gamespot, Microsoft senior director of product management Albert Panello revealed that vertical orientation will not be supported for the XBox One:
We don't support vertical orientation; do it at your own risk. It wouldn't be a cooling problem, we just didn't design the drive for vertical. Because it's a slot loading drive, we just didn't design it for both.
Panello also claims that 80% of 360 owners keep their consoles horizontally. Do you?
Is your 360 horizontal or vertical?