Timothy Burke has an interesting meditation on the difference between plagiarism, remixing (in Lessing's sense) and allusion:

http://weblogs.swarthmore.edu/burke/?p=179

"However, I don’t think Lessig leaves me any grounds for seeing a relation between the active plagiarism of Viswanathan and the derivative character of Paolini’s writing, as one example. If all creativity is essentially remixing, then what do we mean by originality?"