The version as we know it today was copyrighted in 1935 by the Summy Company. The copyright is set to expire in 2030. Warner Chapell purchased the copyright in 1990 for $15 million. The value is estimated at $5 million. Warner claims that unauthorized performances of the song are illegal unless royalties are paid to them. 

• For the movie "The Corporation" the producers claim that Warner/Chappell were going to charge up to $10,000 for the song to appear in a film.

• In the first season of the show Sports Night, Dan Rydell is told that his company will have to pay $2,500 in legal costs because he sang "Happy Birthday" to his co-anchor Casey on air.
  

(Warning: Sarcasm Ahead)

If you have seen someone singing Happy Birthday in a restaurant, a park, or at a school, you should tell ASCAP so that they can arrange for a license. If you are an offender, you should apologize and offer to pay whatever is due — a nickel, a quarter, a dollar — whatever ASCAP demands.

There are many ways to get in contact with ASCAP:

Email
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Mailing Address
ASCAP - New York
One Lincoln Plaza
New York, NY 10023

Phone
1-800-505-4052

It would also be a good idea to keep the song's owner — Time Warner — in the loop. Here is their contact information:

Mailing Address
Time Warner Inc.
One Time Warner Center
New York, NY 10019-8016

Phone
1-212-484-8000