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According to Vocaloid official site (old ver)
See also Is there anyway a Vocaloid program can work on a Mac?

"Yamaha understands that Macintosh is a very important platform for use in creating music and would like to develop a Mac version in the future. However, the definite release date has not yet been decided upon."

Alternatives are the iVocaloid a the portable version of the software released for the iPad. And the Garageband software Macne series(fans refer to as Macloid), this is a synthesizer program currently with six characters with different voice banks.

See also, UTAU wiki on commercial products and Vocaloid wiki on Macne Series. Recently there was a UTAU release for MacOS, see the official site Utau-synth(in Japanese) for details and this article entry from Vocaloidism.