In the past two releases of Mac OS X, we've seen the baseline support for legacy machines hiked a notch higher each time; for Leopard, eliminating slower G4s from the working list, and for Snow Leopard dropping PowerPC support entirely in favor of the newer Intel models.
Now, with the developer preview of Mac OS X 10.7 Lion in the wild, the next frontier for obsolescence is set -- the new OS drops official support for Core Duo and Core Solo-based Intel Macs as well.
We have heard from some testers that they've successfully installed and booted Lion on the older unsupported machines, but your mileage may vary. Update: Our correspondents were confused about the requirements, and in fact were using supported machines.
Why the shift? Apple's not saying yet, but the machines based on these 32-bit Intel CPUs may not have the horsepower or addressable memory space to support Lion, or Apple may be pushing towards a full 64-bit OS and kernel (which might cause some issues for hardware drivers and peripherals). In any event, if you've got a first-generation Intel Mac that's more than four and a half years old, you may be staying with 10.6 Snow Leopard (or Leopard or Tiger, for that matter, if you haven't updated).
Machines with the newer 64-bit Core 2 Duo and later chips are almost all good to go with the new cat in town, with one exception in this preview release; "late 2006" iMacs with Core 2 Duos are not yet supported.
Keep in mind that the exact system requirements and supported models may change between now and the official release of 10.7 sometime this summer.
[hat tip to ZDnet]