Lucien, I mispoke when I said that perhaps copyright duration should be dependent on the medium -- I meant the type of work.
Shivohum, thanks for joining in, I was beginning to think Lucien and I scared everyone way.
As for "to promote the progress of science and useful arts" -- commonly referred to as the "preamble" to the Copyright clause --there is much dispute as to whether it should be read as a substantive restriction on the power of congress, or if it is merely guiding language. Further, a strict reading of the language could mean that congress can only enact copyright protection for the precise time that is needed to promote the creation of works. Any greather time would, under this reading, be outside the power of congress to enact. However, it also makes sense to read the preamble as merely dictating a purpose, and not restricting congress' power. In short, is this language meant to be a restriction?
Also, it could be argued, though I suggest only quite weakly, that extending copyright terms retroactively could promote the progress of science (which, btw, has long been interpretted broadly to include creation of art). To use the copyright devil as an example, let's assume that a janitor as Disney finds an extremely old Mickey Mouse cartoon, that even predates Steamboat Willy (sorry if my Disney history is off). The Disney lawyers determine that the copyright on this newly discovered cartoon lapsed last year. Further, the quality is so low that it would take a substantial investment to restore the print. Disney would have no incentive to do any of this, because they probably wouldn't be able to turn a profit on the cartoon, and thus may let it continue to collect dust. My extending the copyright term retroactively, congress would provide Disney an incentive to bring to market this Mickey Mouse cartoon.
As for the CTEA, it was supposedly passed to match the copyright term for the European Union (I might be getting the facts slightly wrong, this is from memory). Apparently, the EU extended the term of copyright, but with the following condition. A member country was required to enforce a copyright from the shorter of (a) the term that would be granted to the work had it been first published in such country or (b) the term of protection granted to the work in its country of origin. The more I think about it, the more I'm certain I'm getting this wrong, but it is well published and you should be able to find information if you're interested. Unfortunately I'm not an home right now and can't look it up myself. However, the general gist is that U.S. works were getting less protection than their European counterparts, so congress lengethened the copyright term to even it out.