The old anarchist adage goes 'property is theft', however few realise that Pierre-Joseph Proudhon meant the property of state, the landlords and the capitalists, rather than the individual. Indeed he saw individual property as "the triumph of Liberty".

The same distinction can apply to 'intellectual property'. Indeed intellectual property presents the added problem that it is meant to be shared, unlike, say, your house, your dog, or your toothbrush. And only by risking its theft through publication, does the right come into definition.
So there is a distinction between the individual rights of authors and those of a large publishing house. The internet has further complicated matters, by emancipating individual authors and other inventors/creators, but at the same time easing copying/theft -witness peer to peer programs and their agonising persecution by music multinationals. Plagiarism, a form of intellectual theft, abounds online, but on the other hand it can be detected in milliseconds, also thanks to the Internet.

Movements such as the Open Source, Copyleft, and Creative Commons, offer another approach, that of organised sharing. However this does not stop others from using such open source raw materials, to create commercial products, and some suspect it thus becomes another form of exploitation, this time with the consent (naive or otherwise) of creators.

Other forms of intellectual property, such as biotechnological and pharmaceutical patents (cf Aids related), are more controversial, and indeed have been ignored by major countries, who brought pharmaceutical giants to their knees. So there is a precedent that the application of these intellectual property rights is subservient to the 'common good'.
Property, ownership, is at the heart of civilisation, and every major ideology, secular or religious, has its own approach, or excuse. It probably goes all the way back to our animal ancestors - we do describe some animals as 'territorial' no? And territory is linked to war, an act of defending this territory. And some wars indeed start as intellectual wars. In a way therefore, the word intellectual property contains a fossil of 'organised violence' - 'what will happen if', so it may be an irony that the UN decided to create a Day, one of so many world days, to celebrate it.