Sorry I'm coming in a little late to this torrent of replies to your post but you may be interested in this post about why "gut feeling" by police isn't always a "gut feeling," but may be supported by fact.I've only had time to skim the article but it looks interesting. However, I would advise skepticism. Once upon a time your faithful author was an interrogator in the military and received a good bit of training on this sort of thing. There is a good bit of truth to it. However, those who wedded to the idea of using facial expressions and body language almost always overstate its effectiveness.
The Naked Face: Can you read people's thoughts just by looking at them?
I have guest lectured at the local police academy a few times and a lot of the lecture is focused on Terry stops. After reviewing the language from Terry itself, (not a hunch, objective fact reasonable suspicion, etc), I have emphasized this: trust your gut, because your "hunch" is usually backed by objective facts that are so subtle, you don't realize it. Once you do realize the objective facts, lay them out in your report.
I would agree that "through my training and experience..." is used too much by people. But really experienced officers notice things that the average lay person (myself included) would not notice. Heck, that's what the Cleveland Police detective noticed in Terry, a suspicious guy pacing in front of a closed store.
Many, if not most, of the indicators they point to are stress indicators - not signs of lying (same problem exists with polygraphs). Many indicators are societal in nature: looking away can be seen as an indication of a lie or it can be an ingrained manner of dealing with those who are more powerful than you. And often the "indicators" which these researchers point to are so fleeting that they can only be seen when viewed in slow motion; however, the eye is assumed to have caught them and processed them (an iffy proposition). Additionally, many people, by training or by experience, easily mask indicators or simulate false indicators; in tests of this the "reader" is more likely to read the answer from the unaware researcher than the person faking it. Moreover, these things are usually fairly subtle; they are the kind of things you probably wouldn't notice if you were making an initial observation but require time and proximity to observe.
Do I think that officers notice things I don't? I surely hope they do. The officer should know that three kids in his area wearing "Michael Vick" jerseys belong to the same set and be able to name it. He should know what certain tattoos mean. He should know that a bulge under the arm or at the hip probably means a gun. He should notice broken windows on cars. He should see the two guys walking a third into an alley as he drives past. &cetera, &cetera, &cetera.
I've no problem with the officer noticing people; I have no problems with the officer keeping an eye on someone because he is suspicious; in general I have no problem with a consensual walk up, "Hey guys, what are ya'll up to tonight?" (although this can become problematic quickly). I even agree with the email's assertion that officers should "trust their guts." All of us do this every day.
However, I cannot agree with action based on gut feeling with an eye toward figuring out the whys of the feeling afterward. Short of an instant action situation (a man running out a bank while the alarm is blaring) the officer must develop his individualized reasonable articulable suspicion of a criminal act prior to reacting. To tell an officer to figure it out later is to encourage the officer to use 20/20 hindsight (or worse): i.e. "I'm sure I saw that bulge under his arm where he had the pistol."