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.
You can click on the Apple in the top left corner and click "About This Mac". This will show what kind of processor you have. If you have an Intel processor then your computer is going to be supported after the patch. If you have a PowerPC processor then you are not going to be able to play.