This article was written by Phillip Ball in May 1st 2008 issue of Nature. I strongly suggest that, if you haven't read it and have access to it, that you take some time reading it. It deals with the issue of the "transition" or boundary or crossover or whatever between classical and quantum regimes.
To understand what the quantum–classical transition really means, consider that our familiar, classical world is an ‘either/or’ kind of place. A compass needle, say, can’t point both north and south at the same time. The quantum world, by contrast, is ‘both/and’: a magnetic atom, say, has no trouble at all pointing both directions at once. The same is true for other properties such as energy, location or speed; generally speaking, they can take on a range of values simultaneously, so that all you can say is that this value has that probability. When that is the case, physicists say that a quantum object is in a ‘superposition’
Thus, one of the key questions in understanding the quantum–classical transition is what happens to the superpositions as you go up that atoms-to-apples scale? Exactly when and how does ‘both/and’ become ‘either/or’?
Of course, there is a very good coverage of the leading candidate that tries to connect between the classical-quantum transition - decoherence. One of the important point of the article is the idea that it isn't the SIZE of the object that is important, but rather the time scale for when decoherence sets in.
Decoherence also predicts that the quantum–classical transition isn’t really a matter of size, but of time. The stronger a quantum object’s interactions are with its surroundings, the faster decoherence kicks in. So larger objects, which generally have more ways of interacting, decohere almost instantaneously, transforming their quantum character into classical behaviour just as quickly. For example, if a large molecule could be prepared in a superposition of two positions just 10 ångstroms apart, it would decohere because of collisions with the surrounding air molecules in about 10−17 seconds. Decoherence is unavoidable to some degree. Even in a perfect vacuum, particles will decohere through interactions with photons in the omnipresent cosmic microwave background.
So that's why we can still get interference pattern when particles as large as buckyballs are used, or that we can still see superposition effects in 10^11 particles, as in the SQUID experiments from Delft/Stony Brook.
The article also pointed out the alternative idea from Penrose that the coupling of the system to gravity (or gravitons to be exact) might be responsible for the emergence of our classical observation. I mentioned this earlier in another blog entry, including the upcoming tests being proposed Dirk Bouwmeester.
A great article, even if only for the wealth of the references given. A highly-recommended reading.