According to a post over on the Blizzard forums, World of Warcraft will no longer support older PowerPC Macs once Cataclysm is released.
Per a notice posted by Jessicalu in the Mac support forums, the patch is due to be released before the expansion:
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.
Likely, this will only apply to a few, as most Mac users own Intel-based systems now and days.
The beta test for World of Warcraft: Cataclysm is currently underway, and a release date is expected to be announced “in the months ahead” - meaning BlizzCon more likely than not.