Urban space is defined by the configuration of the various opaque barriers which restrict our lines of sight. However, two technological developments will transform the concept of urban space within our lifetimes: terahertz sensing and organic LEDs.
Terahertz sensors can see through masonry, plastic, wood, ceramics and clothing (although they cannot see through metal or water). They are currently being developed for use in asymmetric warfare and counter-terrorism, where it is necessary to distinguish between civilian and combatant. Inevitably, however, like infra-red technology, the price of terahertz sensing will drop to the level at which it becomes commercially available. If there is sufficient consumer demand, the technology will then become ubiquitous. At that point, many of the restrictions to our current lines of sight will evaporate. Unless your house or place of work is made of metal, you'll be visible at all times.
'Organic', or polymer-based transistors and LEDs enable the construction of wafer-thin television screens upon the contours of any surface. Any wall, any roof, any floor, interior or exterior, will become a potential site for a television screen. If television is indeed a device for transporting the mind elsewhere, then every line of sight will intersect devices capable of transporting our minds elsewhere.
Perhaps urban designers will respond to terahertz technology by building metal shielding, or by constructing baroque three-dimensional water structures around solid architecture. Or perhaps government legislation would deem such measures inconsistent with the fight against terrorism. Given the potential for advertising, it seems unlikely that government would legislate against the spread of ubiquitous organic-LED billboards.
It seems that the dystopic visions of Philip K. Dick will be coming to an urban space near you.