Some additional information about how the copyright may be challenged is here:
http://www.snopes.com/music/songs/birthday.asp The general rule of thumb for expiration of copyright is that anything published prior to January 1, 1923 are in the public domain. That assumes that all extensions available since that date have been obtained. Current copyright law gives an enormously longer duration of protection than the 56 years envisaged when the Happy Birthday thing hit the streets, and protection will doubtless be extended retroactively in the future as it was in the past, as immortal corporations bribe congress into extending it a century or so down the line when the public might get access to current material.
The original intent of copyright, like the patent system, was to encourage the production of new work by giving the creator a monopoly that was limited in duration sufficient to make it worth his while to create it. Eventually the work was to become the property of the public. Would you think it was worth your time to write a hit song even if you knew it would not still be paying royalties 70 years after your death?