;-) As I understand it, Amiga OS 4.0 will be released with a list of supported hardware. The philosophy at this time is that they would prefer to support a narrow range of hardware well, than a wide range of hardware, badly.
A number of Amiga.org members already have the AmigaOne, and are happy with them. OS 4 hasn't been released just yet, but it's just a matter of time.
There is a version of OS 4 for the existing PPC processor cards, such as the CyberStorm, etc. That version will be released first, with the AmigaOne version to follow. As my A4000 is equipped with a CyberStormIII, I'm champing at the bit for OS 4. After I am able to run it for myself, I'll decide whether to order the AmigaOne hardware as well.