My three-year old daughter refused to wear a jacket for most of this winter. She did insist, however, on always wearing her hat and mittens.
As frustrating as this was as a parent, she was perfectly comfortable - as long as her extremities were covered.
The same holds true for eye contact during a presentation. Focus the majority of time on "the extremities" - the people at the edges of the room, who are the most likely to be ignored. When you look people on the edges, you include everyone and make everyone feel comfortable. When you focus mainly on the middle, you exclude the folks on the side - AND make those in the middle feel uncomfortable.
From the Green Room: Cover the extremities. The most important people to look at are the people at the farthest ends of the room. By focusing on the edges, you include everyone.