cbrown, I think the OP was talking about running RouterOS on the bare metal hardware, with no Mac OS X installed, and using the KVM hypervisor (built into x86 RouterOS Linux kernel) to run either other virtual instances of RouterOS or other (Linux) guests on top of RouterOS.
Does RouterOS support the ethernet chipset(s) on past and present Minis?
I could see wireless working and being controlled directly by RouterOS if you have a Mini with an Atheros chipset, or if you replace the wireless module (which, I believe, might be MiniPCI or MiniPCIe on many models) with one based on Atheros (I believe some Minis use Broadcom instead).
EDIT: Actually, even I misread OP. He was talking about installing a Linux distro (CentOS) on the bare metal, then running KVM on top of that and running RouterOS as a KVM guest. I suppose that works too, and gives you a much broader set of supported hardware. I'd be very interested, though, in hearing from anybody that has installed RouterOS directly on a Mac Mini without another virtualization layer.