Since there are so many more perceived threats, and since gaps and weaknesses in the domestic security net are so well documented, it seems prudent to have a look at technology that can plug those gaps. Technology to the rescue: blimps ahoy!
Border radar arrays can't see into every ravine and gully, where bad guys could get into the country, undetected by the existing technology. Cruise missiles can do it, too, if the ravine is straight enough, long enough. Those incursions need to be picked out with a line-of-sight technology, like from above. Satellites take great photos; but they can't see everything, all the time. Airplanes cost a lot of money to keep in the air, have limited endurance, and require the pilot's attention. They nearly all have severed downward-visibility limitations, too.
So, how about some really old, proven technology? A recent AP article as much as asks, "How about a blimp?" Blimps can stay on station for an entire daylight (or night) shift (or, unmanned, months); they can be deployed to areas where they're most-needed, and relatively quickly. They're silent, at altitude. They require only small crews (and unmanned blimps are available, for long missions, too), and have the ability to travel unpredictably, so that bad guys can't count on any particular geographical window of opportunity.

Blimp Companies Ready to Strut Their Wares

CargoLifter AG, clobbered by post-September trauma, may have found a new reason to be viable. In addition to its heavy construction-transport models, the company is planning to adapt its heavy-lift technology to a long-term surveillance machine, that would spend most of its time tethered above the sensitive spot. CargoLifter is affiliated with Boeing.
Lockheed Martin Naval Electronics & Surveillance Systems says it has designs that could readily be produced, in case somebody else's design looks like it might get funding.
Zeppelin, the name in the business for a hundred years, has both trainers and passenger models ready to be adapted. Even Goodyear has dirigible designs of its own.

NORAD has a check waiting somewhere

NORAD says it would like some machines that would float at 70,000 feet or so, to stay out of weather and commercial traffic (and U-2 traffic, and B-1 traffic, and...); it should be filled with helium, so it doesn't do the Hindenburg thing. They should be able to look for not just low-level traffic, but also ICBMs, stay aloft for months, and cost no more than $100 million... each. They need ten of them.

FMI: http://www.boeing.com/defense-space/military/phantom/phantom.htm,  http://www.cargolifter.com/2002/repository/splash_e.html, http://www.lockheedmartin.com/, http://www.zeppelin-nt.com/