Universal Mac OS X?
It looks like it’s possible to build a Universal image for Mac OS X, with a few caveats. Most binaries included with the Intel 10.4.x are in fact Universal builds, which allow both PPC Macs and Intel Macs to use them. This includes the kernel, most kernel extensions, the system libraries and frameworks; the Intel 10.4 even includes the bootloaders for both platforms.
The methods and madness are documented by Andrew Mortensen at his website at http://www-personal.umich.edu/~admorten/CreatingUniversalTigerWithRadmind.pdf