“What if Apple were to build the equivalent of OpenStep for Linux? That is, take the whole of OS X and build it around a Linux kernel instead of Mach…”
I personally, think it would be pretty cool if Apple kicked Mach’s ass and went with the Linux kernel, because they give Micro$oft a smack in the face, by having their current biggest compeditor, on the desktop, be Linux-based. I find it highly unlikely though, because they are a proprietary software company, and I don’t think they like the “once free, always free” nature of the GPL.
If Steve are going to kick Mach out the tree (apple tree get it?), then I think it’s more likely, that they will just go with the regular FreeBSD kernel, after all, their system are already heavily based on FreeBSD, and it’s a great kernel with over 20 years of development put into it.