A Security Blanket, but With No Guarantees
New York region is in extraordinary state of heightened security as US wages war in Iraq; heavily armed police officers are outside prominent buildings, police boats are combing waterfronts and trucks are being inspected at bridges and tunnels; some precautions will cease when war ends, but many will not; in 18 months since World Trade Center attack, significant and lasting changes have been made in region's security landscape and more are in works; detailed look at some security and warning systems in place or being planned; photos; chart (L)