Obviously
Anyone who didn't see this coming was just being naive. Apple said when releasing 10.5 that they believed many people would be sticking with leopard far longer than until they released an updated OS. If the whole goal of the OS is to be super light and highly optimized for multiple cores, its logical that it would be exceedingly more effective to substantially limit the hardware you support. It totally flies in the face of stripping down the size of the OS as well as using it as a stepping stone for 10.7 which will certainly not support 4-6 year old PPC computers.
The only people with a small right to complain are the dual-core G5 owners, a very very small demographic of mac users who, while clearly desirous of a very powerful pc, aren't desperate enough for more power to be updating on the average 2-3 year cycle. Remember, it will have been 3 years since the dual G5 days when this OS ships. Since it would be a ton more work to try and optimize multi-core tech for vastly different architectures, and the fruits of such labor would benefit so few, why is apple expected to do this?
Finally, the assumption at 10.5 support will stop when 10.6 releases is absurd. There is every reason to believe that 10.5 will continue to see incremental updates after 10.6 is released, especially since Apple seems to be viewing 10.6 as a professional\developer only upgrade.