Here’s a couple of posts on the negative-temperature experiment that was in the news last week:
Uncertain Principles: What Does “Negative Temperature” Mean, Anyway?
Built on Facts: Less Than Absolute Zero
The upshot of all this is that many of the stories were generally over-hyping the negative temperature aspect of the experiment, some to the point of simply getting it wrong that somehow this was a new and earth-shattering result. A spin system was put into a negative temperature state way back in the early 1950’s, and (as Matt points out) lasers have population inversions in them as well. One should note that you don’t get these negative temperatures via straightforward thermodynamic means — you have to rig the system to do so, meaning they don’t happen spontaneously.
I heard this done very badly on the science magazine podcast. I listened, and screamed “lasers, lasers, lasers”. Also, temperature is defined for large things in equilibrium. If stuff is redefined, it may be fun, but the words are being used in a different sense to everyone else.