I have mine in a horizontal case with minimal openings and mine gets hot as well. Mine is the SGH-I777.
I suspect mine is getting hot because I was using dxTop, a launcher replacement (that I really like BTW); so I turned off my alternate launcher and I will see how it acts.
My Infuse did the same thing, and I griped about it being hot and having poor battery life as well, AND... I was using dxTop on that as well. So... dxTop? We'll see...