The DMA issues brought up here really don't affect AmigaOS4. If you have a Micro AmigaOne (Mini-ITX), there doesn't seem to be a DMA issue, and if you have an AmigaOne XE (ATX) there is a PCB mod to make everything happy, or you can use a PCI IDE card and USB hub and avoid the hardware patches. AmigaOne SE (ATX) was the original board which I haven't heard of any PCB mods for, but it also uses an older southbridge revision that may not take the XE mods or something like that.
If you want to use Linux, there is a patched 2.4.something kernel which works as I understand, and someone is working on patching 2.6.something as well for the special DMA considerations to work. All will work fine with DMA disabled in Linux.
There's a couple other things in the works but there aren't many details available yet. ACK is working on a new classic Amiga PPC accelerator, Olegil has a project (and I believe he is capable so long as he keeps the motivation going), and Garry Hare at Amiga Inc. has mentioned there's some other device that he can't say anything more than that about due to NDAs at the moment.