I'm not exactly sure how it works, but I think most of the newer install methods will put the extra (non-Apple) drivers on an EFI support partition that isn't touched during system upgrades, to keep the install as pristine as possible. This also allows you to install from a real OS X disc.

In my personal experience, the Mini 9 didn't have any trouble with the 10.5.7 upgrade. I had to boot into safe mode and run the driver installer again, because the new update apparently affected the video card driver, but that was it.