We're looking into the possibility of creating a Mac version of Pianissimo. In the meantime, there are devices like the VMachine, which allow you to run PC-compatible virtual instruments (like Pianissimo) on your Macintosh.
We've also been told that you can download the VMachine host software and use it to run PC-compatible virtual instruments on your Mac without the actual hardware device. A beta version of this software should be available by March 2009. You can try that here.