What's basically happened in the US is that, for things published from 1978 onwards, you have a "life + 70" copyright law, just like most of the rest of the world.
Where it differs from other countries is that, almost entirely due to Disney's political lobbying power (which is considerable), you also have a "publication date + 95 years" protection for things published between 1923 and 1978.
What this means is that nothing new will enter the public domain in the US until the year 2018, and things won't start entering the PD under the normal "life + 70" law that everyone else uses until 2048! I wouldn't even get too excited about the 2018 date. What all observers gloomily agree will happen is that, as that date approaches, Congress will simply extend the "publication + 95 years" protection to something longer.
It really is a pretty shocking state of affairs. Effectively, the public domain has been "sealed off" to new work in the US for the foreseeable future. Things might start working normally again in 2048, but that's a long way off.