nettime: There is a good description of art appropriation from the Detournement movement in 1950s to intellectual property being the “Oil of the 21st century”:

In a post-industrial society production is no
longer confined to material goods (such as steel
and coal) but increasingly extends to immaterial
goods. However, a significant difference exists
between the two: immaterial goods like knowledge
and information can be reproduced without
impairment, which is not the case with material
goods. But in order to be able to function within
a value chain, the distribution of these
immaterial goods must be restricted - namely with
the aid of patent, copyright and trademark law.