Have the copyright extensions actually been upheld in court? Nothing stops a legislator from passing an unconstitutional law, but if nobody fights it then it won't get struck down.
Lawrence Lessig was lead counsel for the plaintiffs (ie arguing the case against the act). He wrote an essay on the case and why he thinks it was lost: http://www.legalaffairs.org/issues/March-April-2004/story_le...
Depends on how you literally minded you read it and how the Supreme court feels:
1) Any copyright or patent that does not promote Science of Useful arts is unconstitutional
2) Only written words are protected, not visual or software.
3) There should be some limitation on the term but it could be way past couple of googols in years.
and so on. The constitution is (un) intentionally vague in a lot of places.
4) Copyright cannot extend past the life of the author, since only the author can have exclusive rights to his writings. I.e. copyright is non-transferable to heirs, corporations, etc.
The most recent extension immediately received a Supreme Court challenge.