Walt Disney's Studio claims of copyrights for Mickey Mouse was due to expire in 2003. This was to be followed in 2009 by many other early Disney's character including Pluto, Goofy and Donald Duck.
According to the copyright law, upon the expiration of the copyright protection, anyone could copy, reproduce and sell any articles covered by the copyrights without paying royalties to the original owner since it has been a public domain.
But, being a very influential entity in politics, The Walt Disney Company along with other copyright holders managed to persuade the American Congress to change the copyright law.
Under the so called The Sonny Bono Term Extension Act, or more famously known as "The Mickey Mouse Protection Act", which was passed on 28th October 1998, the copyright protection for Mickey Mouse and other Disney's characters gets an extension of 20 years.
This means that Mickey Mouse will not become public domain at least until 2023.