AppleInsider is speculating that Apple may drop support for PowerPC in Mac OS X 10.6 (estimated to come out sometime in 2009/2010).
Looking ahead, those people familiar with Apple development cycles speculate that Mac OS X 10.6 will exclude support for PowerPC-based Macs entirely, requiring that users have one of the company’s Intel-based systems which first began making their way to market in early 2006.
One thing we knew – the life of the G3 was at an end in terms of software support as it had been a long time since Apple had shipped a G3-based machine. I’m not basing this on speculation, I’m basing this on the Release Notes which were leaked months and months ago.
I’m not keen on speculation with regards to this sort of thing. We have no way of knowing really what Apple will or won’t support. I would be surprised if they were going to drop everything slower than 867 MHz because it’s such an arbitrary number and there were a lot of machines just slightly slower than that which would obviously be fine. It’s a reasonable assumption however that the option-Install method will still be available to allow installation on unsupported hardware.
The presence of absence of a decent graphics card would be a much better indicator.