I happen to have, in my hand, a GPS unit capable of both vertical and horizontal display. My cell phone is a Garminfone which is basically a GPS unit that has a somewhat smart phone built into it. It's advantage to other phones' navigation apps is that it actually uses the GPS satellites for navigation rather than data over your cell signal so it still works way out in the boonies. Because it's also a smart phone, it will change the orientation of the map depending on which way you hold/mount it. I've never used it vertically, but it does show more of what is ahead of you if held that way. I could see where this could be advantageous. I may readjust my ram mount and try it that way and see if I like it.
Unfortunately I don't think they make that phone anymore. It isn't real great at being a smart phone, so it never really caught on. It is, however, a pretty good GPS unit.
Mom, I'm not wearing my helmet only for the purpose of taking this picture and
the background was digitally blurred to make it look like I'm going really fast.