Well, i agree this currently doesn't have 100% to succeed, but anyway it's really an interesting task. There are several things which say that it's probably possible (currently i think chances are ~75-80%):
1. There is a project called XPostFacto aimed at running OS X on earlier Mac models, officially unsupported by Apple. The project consists of modified BootX loader and a set of kexts (kernel extensions = hardware drivers) which are put into the system in addition to supplied ones. I see an end result of my project to be something like this.
2. I've studied the source code of xnu kernel. There's nothing inside which really prevents it from booting on non-Mac system except one check for "mac-io" device presence in the OpenFirmware tree. However, this check has a hole and can be bypassed.
This means OS X kernel can run without being recompiled. As to recompiling the kernel, well, i beleive Apple's programmers are not so stupid and they've excluded some parts from opensource version of the kernel.