Both Cafe OS and IOSU are most likely something custom by Nintendo. There's almost no resemblance between Cafe OS and Unix, and only superficial resemblance to Windows NT (Nintendo probably read Windows Internals before writing it). IOSU does expose a userspace interface sort of like Unix (/dev nodes, read(), write(), ioctl(), etc.), but internally it's clearly a custom microkernel. A (defunct?) company called BroadOn wrote the Wii IOS, but IOSU is pretty different, so Nintendo probably redid it from scratch.
But I thought that Luke Dixon said that Cafe OS was based EXCLUSIVELY on Linux?