I have an iPod touch, so I'm familiar with the interface. In some ways it is MUCH simpler to tell scrollable vs non-scrollable. At the same time, it isn't really a huge issue on the iPhone, but yes there is a difference.
The iPhone home screen may be the best example. I've got a screen full of icons, and nothing really tells me that there is another page of icons waiting (unless I look at the very small dots at the bottom of the screen).
On the ZuneHD, I have two cues. 1) the gap at the top, and 2) the last item is partially cut-off as a hint that there is more to come.
If zune had just had the list without the gap at the top, you would only know you've reached the top by the end of scrolling. The negative space, I find, to be a nice way to say that you're at the top or bottom, without adding another UI element like a bar or something.
On the iPhone, they've accomplished this with an 'over scroll' which shows me just black space and then springs back to the icons. Try it yourself, on your home screen scroll too far right, or too far left.
The Zune has the same effect, though it isn't necessary because they've added the negative space.
The Zune uses negative space in applications as well, though I feel they could have done a better job, I'm sure it will improve.
As for the negative space on the right, on the Zune, you can use that to control volume in the music player (though it brings up a function screen which I'm not a huge fan of), the use of a large negative space makes it so I can control the volume without having to look at the device (changing tracks can be done with a swipe, so no look for that is really good too). For some reason he button which is located at the usual volume spot on the iphone doesn't change the volume. I'm not sure why.