Why does yesterday's salt-based beer chilling tip work? I'd assumed it was simply because the salt changed the melting point of the ice, allowing water to get colder than it normally does, but Midwest Product says there's more:

The presence of the salt does allow the water temperature to drop below 32 degrees, but this trick also works in part because the salt speeds up the melting of the ice. The phase transition from solid to liquid absorbs heat, which helps keep the water extra cold.

That's why calipygian's trick is related; it simply uses the transition from liquid to gas rather than from solid to liquid. Just like melting ice, evaporating water absorbs heat from its surroundings (in his example, the warm bottle).