IncGamers has some info they say is from the Blizzard forums, indicating that Blizzard is planning to drop support for PowerPC-powered Macs in World of Warcraft sometime before the release of the upcoming Cataclysm (thanks Ant). Here's word:

A little forewarning to those of you that play World of Warcraft on these systems;

Shortly before the release of Cataclysm, a patch will be released in which PowerPC processor based Macs will no longer be supported.

This update indicates the removal of PowerPC support from World of Warcraft.

Attempting to run the game after the update will bring up a message stating the hardware is no longer supported, and the game will not run.