Wow, I think it's funny that my 3 year old thread has been dug up
What I've come to realize is that our expectations of thinness is perpetually changing. For the iPad 1, it was amazing how thin and light it was. Then the iPad 2 came out and it was unbelievable how thin it got. I'll even admit that I voted that I thought the iPad 2 was thin enough. Now the iPad air is out and now people are saying "Ok NOW it's thin enough; I don't need any thinner"
The same thing happened with the iPhone. When the iPhone 4 came out, I never imagined needing/wanting a thinner phone, just make it better. Then the iPhone 5 came out and I was like "Ok, yeah this is pretty great, NOW it's the perfect size". Now I'm totally ready for the iPhone 6 and can't even imagine it getting any thinner. But I'm sure that it will...
I think as long as products continue to gain functionality and at least maintain battery life, we will always be happy with thinner products... Of course, technically there IS a limit.... and as we approach that limit the changes will be less and less dramatic...