I work in a large open space but often I'm the only one there. I find that the amount of time I spend slacking depends more on the nature of my work and on my personal condition than on wheter there are other people around.

When I'm tired and I have lots of dreadfully boring things to do, I end up reading HN. When I have a challenging task, and I'm well rested, I get into the zone and everything around me fades away and I'm really productive. The surrounding (people or no people) seems less important.