I think it's a classic case of 'A CPU does not a computer make.' I'm really not terribly clued up on this but it is a different board, and don't forget that even different Amigas needed different versions of the Amiga OS, so straight away it's obvious that it's not simply a case of sticking the install disc in the drive and hoping for the best.
It wouldn't be impossible to port it though. And Macs are such an obvious candidate for the PPC Amiga OS that it's bound to have been considered. I don't know about Apple's feelings regarding other OSs running on their Macs.
But the legal issue is there, I'm afraid. IIRC (feel free to correct me if I'm wrong anyone) it is against the license agreement of OS4 to run it on anything except licensed (ie. AmigaOne, MicroA1) boards, and the OS is designed not to run on anything else - although it might be coaxed into doing so, either with BIOS reflashes or cracking the OS itself, both of which probably violate the license agreement as well. Which is lame, but that's just my opinion.
It would be ace to have a "Powered by G5" AmigaOS though ;-)
