William Stepp does an admirable job rebutting
Helprin's deeply flawed reasoning in arguing for perpetual copyrights. You might also want to check out the response
from Iiya Somin over at Volokh.
Somin reiterates a thought that always comes to my mind on this issue that cuts right to the heart of the matter: Imagine the consequences of giving Shakespeare's great-great-great-great-great grandchildren the power to end distribution and performances of his work for all time. It would also prevent the creation of any derivative works (i.e., no West Side Story, RAN, and thousands of other works based on Shakespeare. All wiped out since they would violate the "property rights" of someone who has been dead for hundreds of years.).
Helprin's idea is not merely wrong - it would be utterly destructive to any semblance of artistic culture.
[Update: Lessig has put together a useful wiki page for rebutting Helprin. Hat-tip James Grimmelmann over at PrawfsBlog.
Thanks for posting these links. I perused Lessig's wiki page, and noted his discussion of Locke. He says, I think correctly, that Locke's view of property was inconsistent with copyright.
However, Locke endorsed copyright in a couple of letters. Unfortunately I don't have the citations and don't remember the book they were discussed in--I think it was one mentioned by Bill Patry at his blog a few months ago in his "book corner" discussion.
If I find the source I'll post it.
So far Patry hasn't discussed the Helprin article.