I am curious: if you have seen any images that have a similar effect, please email me.
8/5/2006: Jacinta Reid sent me two suggestions: the classic triangle illusion, and an Islamic design. The Islamic design certainly has the effect on me, the triangle illusion less so, though the white triangle has a disturbing tendency to fade if you look at the figure off-center.
When I was studying fine art, this was discussed, and as I understand it, when we are confronted with an image that has a very even balance and distribution of elements, our eye and brain cannot find a dominant subject to focus on, so reverts to simple 'sort and group' routines. As each group of elements, including the negative space, is selected and compared in turn, the other parts of the image are disregarded and effectively become less visible. Negative space and perspective can also 'pop' in and out, like looking at a Necker Cube field or a candle-stick/face image. This response is all largely involuntary, and the lack of control can be disconcerting.