Here is a quick note on the upgrade process for the Mini 9 to OS X 10.5.7 — It was smooth.
Here is the process I used, in the My Dell Mini forums.
Before the upgrade my machine had DellEFI v1.1 installed. Unfortunately, there was absolutely no time during the loading of the OS to type in any boot options. It took some digging around, but this may help someone else: To boot into safe mode on any Mac, hold down the shift key as the machine powers on and goes through boot. Keep holding the button till it hurts — realistically you can let go once you see the Apple logo. I, however, held it down till I saw the login screen.
Other than that, I followed the directions pretty much to the letter. Well, I did do a full system backup using SuperDuper before I did anything. I spent too much time configuring this device not to have a back up. The back up itself went into a sparse image file on a USB drive. The backup process doubled the amount of time it took to do the upgrade. However, it was a good insurance option.
I did install DellEFI 1.2a5 and Monolingual. The first one gives you a better boot prompt — at least you have time to stop the boot process to enter some boot options. The second one is open source tool to remove all the extra language packs that applications come with. I was able to free up about 500megs using the tool.
I am hoping that my big machine is this easy to upgrade.