Can any liquid food be beaten into a mousse? Hervé This discovered that one can make chocolate mousse with chocolate and water only.
This recipe is shown in this MasterChef video, and also detailed here.
Essentially, one melts chocolate with water, then whisks it over an ice bath to incorporate air bubbles, and after a while the mousse is formed.

Is this property unique to chocolate? Would this be possible, with, say, a strawberry purée?
Chocolate is a solid at room temperature, strawberry puree is not, so I strongly doubt that the strawberry would result in a foam.
The reason chocolate would form a solid foam is that it is largely composed of a two substances - sugar and fat. Together with the air these can form a solid of fats (similar to whipped cream) with microscopic sugar crystals helping keep it in place.
The only thing a strawberry puree would have that might reach similar consistency is the sugars. If you were to heat to a high enough heat that the sugars polymerize crystallise (like in candy), and whisk, you might get a structure like a mousse, but it would be crunchy.