* * STEINWAY & SONS unveiled its 500,000th piano in Carnegie Hall at a recital in which 24 pianists took part, including such names as Peter Orth, Joseph Kalichstein, Christopher O'Riley, Grant Johannesen, Ruth Laredo, Alec Chien, Ilana Vered, Cipa and Misha Dichter, Lazar Berman, and Alexis Weissenberg. The recital was a striking demonstration of how different the same instrument can sound under different fingers and feet.

"The new instrument, designed by Wendell Castle, is a chunky, angular, colorful, rather odd-looking piece of furniture," said New Yorker Magazine, "made of rare East Indian ebony and dyed Swiss pear, and trimmed in bubinga wood with bits of brass and Gaboon ebony." The signature of every living Steinway artist has been engraved on it - more than 900 signatures, in slanting bands.* * VALDIMIR DEREVIANKO, former soloist with the Bolshoi Ballet, has been dancing on his own in the West since 1983. But unlike Barishnikov or Nureyev, who fled the Soviet Union for artistic reasons, Derevianko left to be with his Italian wife, Paola, whom he met while she was a student at the Bolshoi school. On tour in 1982 in Italy, he just decided to stay.

Noted for fastidious technique, he is a protege of Vladimir Vasiliev, with whom he danced recently at the 250th anniversary of the San Carlo Theater in Naples. Derevianko is now a member of the Zurich Ballet, has guested with many prominent European companies, and many invitations lie ahead.

* * IRVING BERLIN is honored in his 101st year by an exhibit at Washington's Smithsonian Institution. Called "There's No Business Like Show Business: Irving Berlin's Centennial," the display, which will continue until January, includes original costumes of famous stars, Berlin's piano, theater posters and sheet music from among his more than 800 songs, also vintage film clips.

* * OPERA NOTES: James de Blasis, artistic director of the Cincinnati Opera, celebrated his 20th anniversary with the company in June. Kewei Wang, a baritone from the People's Repuublic of China, won the $10,000 top prize in Baltimore Opera's 25th Anniversary International Vocal Competition. Second prize of $7,500 went to mezzo soprano Luretta Bybee, third prize $4,500 to soprano CeCelia Chaisson.