When Apple announced Mac OS X Snow Leopard, it was met with a lot of fanfare. The refresh for OS X promises a lot of nice enhancements to the system, including a lot of functionality that is specific to newer generation hardware, such as multi-core processors. What is less mentioned (in fact, not at all yet) is what hardware will no longer be supported.
According to some earlier insight, Snow Leopard, due in roughly a year, will no longer support the PowerPC architecture or systems based around it. Effectively, Apple at this point is only supporting Intel processors for the system, which would pretty much rule out all previous-generation Macs from being able to run the OS. As the software is still in development and the notes are only from a developer preview, this could easily change. Eventually, however, they will have to drop support for older hardware.