I downloaded no less than four ISO images of OS X 10.4 from usenet- and all of them appear to be 'modified' (read: hacked) versions intended to run on any Intel hardware (other than Apple's). After a long afternoon of burning and swearing (and playing Super Mario Galaxy) I decided to see what the 'disk tools' could do.
Since the only valid ISO I have is for 10.5, I booted to that DVD. I found I was able to make an image of a HD and save it to another drive. The 60GB HD that was originally in the Mac Mini was a clean install of OS X 10.4.9; I attached this via a USB drive and made an image of this (in the native Apple DMG format) on my firewire drive. Later I restored the image over the internal HD in the Mac Mini and viola- I had a working copy of OS 10.4! The imaging/restore took about 2 hours (laptop SATA via USB to firewire and then back to laptop SATA)
So all is happy and good; except my other firewire and USB drives appear to be hosed! I guess it is time to invest in a new external drive (500GB is usually on sale for $140).
I finally was able to install VMWare fusion; this works very well on a Mac. One very nice feature on the Apple version is the 'Harmony' option- this allows a window from the guest OS (Windows XP, for example) to appear as a window in the Mac OS without all the VMWare boxes, buttons, etc.- so it looks like I have an IE 7 window running on my Mac! :o)
I still need to experiment with Harmony but it looks like it could reduce my dependancy on my PC for many things- just not for gaming as the Mac Mini uses the GMA950 video 'accelerator'... :o)