@seehund, bhoggett:
According to Hyperion, OS4 has been written to sit on top of a HAL, which should make porting to different PPC motherboards less painful than a large-scale rewrite. However, these HALs have to be written. I don't think for a minute that all PowerMacs have the same chipsets, for example a "candy" iMac is probably not going to have the same NB controller as an eMac, or one of the server G4 Macs. That's even assuming you only intend to support "new world" PowerMacs. Each new northbridge needs a new HAL, and the HAL is such a fundamental part of any OS that you have to be 100% certain of its stability before you can say it's done. To me that means that each new supported mobo adds at least one more month until OS4 is released.
In a way, it's a smaller version of the argument against OS4 on x86: which motherboards do you support? right now it's been decided to pick just one, just to get the ball rolling. I would have personally been much happier to be able to pick from at least 3 different models, but I understand why for now at least my choices are Take it or Leave It. It's not a conspiracy, it's a necessity.