The hi-end G4 model of the AmigaOne's CPU card can be upgraded. To what nobody knows yet, but it's supposed to be the standard interface used on the PowerMac.
If anyone wants to get into the PPC line, just get a used PowerMac. You can get an entire system for $800 and you can always run PPC Linux if you want, like SuSe or Yellow Dog. Or stay with OS X.