As an aside to Eldred...

I was sitting here musing about copyrights yesterday, and checking through last year's Eldred Supreme Court case, when I noticed in Eldred's brief that the big name lawyers (Lessig, Sullivan, etc.) focused on the "for a limited time" language in Art. I of the Constitution's grant of power to congress to establish patents and copyrights, but the purpose of copyrights was not the main focus.
Certainly every case is filled with strategic decisions, but it seems to me a productive course would have been not to primarily argue that the constitution's "limited time" language, which is certainly vague and clearly open to different interpretations, didn't mean 70 years but rather that extending copyright protection to life of the author plus 70 years was contrary to the purpose of copyrights as originally understood and as previously defined by the Supreme Court. The Supreme court's opinion (Eldred v. Ashcroft, 537 U.S. 186) certainly cited repeatedly a seminal case, Mazer v. Stein, 347 U.S. 201, 219 (1954), and its statement that:

"The economic philosophy behind the clause empowering Congress to grant patents and copyrights is the conviction that encouragement of individual effort by personal gain is the best way to advance public welfare through the talents of authors and inventors in 'Science and useful Arts.' ".

However, in the end it may not have mattered.