On both computers and cell phones held horizontally, there is on ongoing battle for the space at the top and bottom. More or less, there is less vertical space on a screen every day.
For this reason, I already turn off many icon and tool bar functions in software and browsers already, but there is still a vertical space shortage.
My wife tends to hold her iphone with the screen vertical when she is playing solitare, but routinely switches it to horizontal as needed.
I use my nokia E7 almost always in horizontal mode, and strongly prefer it.
I faced this same question when I had my own web site made, and only wanted links on the left side of the page - no links at all at the top or bottom at all. Interestingly, a number of the templates out there make this quite difficult, but I sort of forced it to happen. That being said, my web site is still a work in progress and has a few bugs.
My suggestion is to also keep in mind that a significant number of people still have relatively modest DSL speed connections (under 1 mbs actual, and significant latency) If a page does not load in 1/2 second or less, I tend to skip the site.