From the official Cyan website:
Fellow Mac users,
Back in February 2004, after trying to get funding for the Mac version of Uru for several years, we announced that the Mac version was underway.
We are extremely disappointed to have to announce today that the Mac port of Uru is officially cancelled.
There were several sticking points along the way, culminating with an issue involving the physics engine used in Uru. The company doing the port found out that there would not be a Mac port of that physics engine. We hoped that alternatives could be found, and we waited for official word with the rest of the Mac community.
Our last request for an update from Ubisoft finally laid those hopes to rest.
We thank you all for your patience in waiting for the Mac version of Uru, and are sorry that your patience, in this case, will not be rewarded.
Rand and the Cyan Worlds Team
PS - We'd like to think that we learned something from the experience. We chose not to use the same physics engine on our current project.