Carl and I saw an interesting educational video on the particle/wave duality of electrons a while ago, and I was finally able to dig it up again: [here]. It's from a movie called "What the *bleep* do we know", and is for the most part, a bunch of newage horsehockey. (Insultingly bad movies physics has a review [here]). Regardless, their animation and explanation of the particle/wave duality is fairly accurate and pretty neat.

It deals with the early physics experiments in which scientists tried to determine the makeup of basic subatomic particles. In this case, electrons. The video explains it very well, right up to the end. It was this final part that I had some reservations about. In the last thirty seconds of the film, the narrator states that if we observe an electron going through the diffraction slit, the electron stops acting like a wave and behaves like a particle. (i.e., it no longer generates a diffraction pattern.)

It is correct in stating that the observation of the electron modifies the outcome, but doesn't go into any detail as to why. The answer lies in the [Heisenberg] [uncertainty principle], which states that we cannot accurately know both the velocity and position of a particle. In a waveform, those properties are analogous to intensity and wavelength (period), respectively.
The actual phenomena behind the experiment is rather straightforward: If we observe carefully which slit it (the electron) is going through (i.e. measure it's intensity at one slit or the other), we cannot know it's wavelength, and therefore, it will act as a particle (hence the particle-esque diffraction pattern seen in the movie). I am no expert in [quantum mechanics], and my grasp of exactly why you must have one observation at the expense of another is not complete enough to explain concisely.

When you ask "but how can a simple observation have such a profound effect?", you're missing the point. It's analogous to asking "what will that gum taste like?". You can't know until you chew it, after which no amount of work can make it unchewed. Because the very act of observation is intrusive it has an effect on the object being observed. It's not like asking the color of my car, which (in our perception) is actually the light rays reflected from my car, not an actual attribute of it. In viewing the light rays not absorbed by my vehicle's paint, we are in no way perturbing the actual state of the car. (Unlike the tasting of gum, as mentioned before).

It is for this reason that I thought the movie was being a bit disingenuous by making the matter more mystical and wonderful than necessary. In the movie, the act of observing the electron traveling through the slit is performed by a camera (or eye), which in our normal day-to-day experience we understand to be devoid of [observer effect] (just as seeing the light reflected from my aforementioned car). In addition, the wording of the narration implies that there is a nebulous "connection" through which the electron is "aware" of the observer. In reality, it's a bit more tangible than that, being more akin to athe "awareness" you have of a police officer when he pulls you over. There's a direct interaction taking place, not some magical hocus-pocus.

If I were given the task to re-write the video more accurately, the detection mechanism would actually stop the electron to verify its presence, and then send on its way (thus demonstrating the true implications of the Heisenberg theory). This is in direct contradiction to the passive/non-involved observer in the film's current incarnation. This, of course, would go much in the way of explaining the dramatic differences between the unobserved and observed experiments. It would also remove a great deal of the "mysticism" of quantum mechanics that is so popular in our culture. (More on this strange facination at a later date)