Ive noticed just casually, using wifi, that the ipad gets a much weaker signal if held perfectly horizontally (flat). Not much more in the way of scientific data on this...
That is because the antenna is mounted to the back of the aluminum ipad case and pretty much used the metal back of the iPad as a big antenna.
When you hold the iPad horizontally the antenna surface to receive and send signal is about the thickness of the ipad. When you hold it Vertically, the whole back of the iPad can absorb the wifi signals being sent to it.
Think of it like this. If your wireless router was a gun, and the signal shooting out of it was a bullet, The picture below is a target. Your chances of hitting the ipad whose back/front is facing you are far greater than the ipad on the far right.