Those laws were NOT made retroactive.
Actually, I'm pretty sure they were, or else many Disney classics would be in the public domain, and they aren't.
I think they went back to the 1920's or so.
Ah, here you go:
http://en.wikipedia.org/wiki/Sonny_Bono_Copyright_Term_Extension_Act
So basically, anything created by a corporation after 1923 is still under copyright.