[bizerl (OP)]

I'm wondering whether "absolute zero" is a calculated conclusion, or an observable phenomenon. If it is the latter, how is it observed?

[Bored chemist]

The calculations show that it's impossible to get there so it cannot be observed.

[bizerl (OP)]

So it's more like an asymptotal target? - We can get ever closer but never quite there?

[evan_au]

Yes - You can get closer and closer by expending more and more energy, but you can never quite get there.
For many years, scientists have observed some characteristics that appear close to absolute zero, including superconductivity and superfluidity.

The current record (in 2012) is less than a billionth of a degree above absolute zero.
This close to absolute zero, Bose-Einstein condensates have been created, where most of the atoms are at their lowest energy level, even though some of the atoms still have some higher energy. http://en.wikipedia.org/wiki/Absolute_zero#Relation_with_Bose.E2.80.93Einstein_condensates

According to Quantum Theory, you can never remove all of the energy from a system.