The words to "Happy Birthday to You" are Copyright 1935 by Mildred J. Hill(1859-1916) and Patty S. Hill(1868-1946). Under the extended 95-year copyright term these words will not enter the public domain until January 1, 2031. But watch for the copyright barons to ask for another 20-year extension beginning around 2015.

The original form of the melody is called "Good Morning to All", and was published in Song Stories for the Kindergarten, Chicago, 1893. This melody is in the public domain, as long as it is used exactly as it appeared in 1893 i.e. with a rhythm that supports an underlying text of "Good morning to all", five syllables. If you use the melody with a divided first note (supporting an underlying text of "happy"), you might get taken to court. Here I am guessing that the melody is the same except for the rhythm of the first note. But I haven't seen the 1893 book, so I can't be sure of this.

T.