So here's a HowTo for updating your Dell Mini 9 to OS X 10.5.7:
1. Get the stuff you need on your disk. The update will bork/break your screen so you have to boot to safe mode. Another added annoyance is that the USB didn't work for me after the update, so having these files on your drive or on a network share is important. Here's the stuff you need.
- If you are on 10.5.5 or lower, use the combo installer. If you are on 10.5.6, you can use the standalone OS X 10.5.7 updater (don't use the system updater, it crashed on me).
- The DellEFI1.1 file
- The AboutThisMac.pkg to fix the About This Mac screen. This isn't necessary but it makes the install look all nice and tidy
2. The instructions for applying the update came from conq at the mydellmini forum. I'm adding a few extra tips that people (including myself) were stumbling on.