When I first heard that New York City had responded to the London bombings with random searches on its subways, my first thought was: "Great. More unconstitutional government surveillance of dubious security value." The Times has a closer examination of the legal issues.
Slippery-slope arguments are, of course, rather suspicious in general. However, these subway searches continue an ongoing slide, over the past few decades, down a long slippery slope w.r.t. accepting government surveillance of its citizens, and I find this incredibly dangerous because the potential expansion of surveillance seems limitless.
The trend has accelerated in recent years, accompanied by an additional dash of terrorism-induced stupidity, in which invasive security "precautions" are embraced without any regard for their effectiveness. The TSA confiscates tweezers, police harass people for taking pictures of tourist attractions, Congress mandates a comprehensive national ID database with "files" on practically every adult American, and the vast majority of Americans just nod. It must be done, in the name of stopping terrorism!
Where do we draw the line? I think that as long as terrorist attacks continue (and they will continue), a majority of Americans will never draw a line. It seems entirely conceivable that at some point in the not-so-distant future, Congress will pass a law mandating random searches of people's houses; and a Federal judge will uphold it (because the ransacking occurs with a fixed frequency that is not up to the discretion of individual officers); and nearly all the members of Congress who voted for the bill will be re-elected.