Before you do the update, make sure you have a version of DellEFI stored on the Mini before you do the upgrade...really, do it.
So you do the update, and it will reboot barely to a gray garbled screen (because it lost the drivers?)
I found the solution here on the 'sproke blog.
It is easy, so I will post it here.
1. Download the installer, don't use Software Update. Install 10.5.7.
2. Let it reboot.
3. At the boot screen press enter (or any key) to get the boot prompt. It should look like this:
boot:
type -x and hit enter
boot: -x
OS X will enter Safe Mode.
4. When in Safe Mode run DellEFI.
5. Reinstall the Mini 9 Extensions and remove custom dsdt.aml file. These are the default options for the DellEFI, take the default.
6. Reboot.
7. At the boot screen use the -x to enter Safe Mode again
8. After in Safe Mode run DellEFI. Install custom dsdt.aml file, which is the default option, take the default.
9. Reboot
Now of course I didn't exactly follow those directions (mine was already broke).
So I booted to safe mode, ran the DellEFI (I used the 1.2a5 found here).
Actually I ran it twice, the first time I selected Custom and checked the remove dsdt.aml.
When it finished, I did not allow it to reboot, and I ran it again. This time the 'create custom dsdt.aml' was already checked. When it finished, I allowed it to reboot and it came up just fine.