mxh178: Most (all?) Mac games (and software in general) released before Apple switched to Intel processors don't work on OSX.
First: That's not entirely true as OS X existed for almost five years (six if you count the Public Beta in 2000) as a retail PPC release, at least publicly (yes, I know about
Marklar). OS X has always existed with portability in mind; in fact, if the rumors are true, Apple likely has a branch of desktop OS X (not iOS) running on ARM processors in their labs.
Second: That depends. If you have a Mac which can run 10.6, you can still run PPC games in Rosetta, with little or no performance hit or compatibility/performance issues.
A lot of older games even harking back to the System 7-OS 9 days have been open sourced, or have open source ports which allow them to work on modern Intel Macs; Hexen, Heretic, DOOM 1-2, Heretic II, Hexen II, Quake 1-3, Descent 1-2, Duke Nukem 3D, and the Marathon Trilogy are the games which immediately come to mind here.
Also, a lot of older, originally PPC-only games were supported with Universal Binaries after Apple's switch to Intel. Halo, Warcraft 3, Jedi Knight II, Jedi Academy, KOTOR, CoD, and CoD 2 are some of the most prominent examples here.
This also of course extended to many key apps for office applications, productivity apps, and creative apps. In fact, if what you said actually was true and everyone's PPC apps stopped working with the release of the Intel Macs, it would be a complete disaster for Apple: no one would buy a new Mac which would break all of your Mac existing apps, no matter how fast it was.