As I understand it, Austrian economists tend to define property with respect to scarce goods; because intellectual property is not a scarce good, they (and, in particular, those at the Mises Institute) argue that an individual cannot legitimately own it as "property." Instead, they view "intellectual property" as something of an oxymoron. More properly, it is a government enforced monopoly on particular ideas -- not property at all.

For much more about Austrian views on intellectual property, take a look at Against Intellectual Property by Stephan Kinsella.