The procedure consists of two steps. First, we have to actually install Aperture. Next, we have to modify the executable to make it ignore the fact that our graphics card is unsupported. I should warn you that this is something that Apple definitely does not support. Should anything go wrong, don't blame me! Your computer may explode. OK, more realistically, Aperture could potentially crash and corrupt all your image files, and then you'd be sorry. Make sure you have a current backup, and proceed at your own risk...
Once you have the Aperture DVD in the drive, you have to get the contents of the Aperture package. The folder with the packages is hidden in the Finder. You can get to it through the Terminal, or in the Finder via the Go -> Go to Folder menu item. Enter /Volumes/Aperture/Packages as the destination -- at least, that's where it was for me.
First install the ProAppRuntime package. That should happen without any coimplaints. Now control (right)-click on the Aperture package and choose Show Package Contents from the pop-up menu. Open the Contents folder, and you should see a file called Archive.pax.gz. Copy it to some temporary folder on your hard drive. Go to that folder and double click, it should unpack. In this folder, a new directory called Applications will appear with Aperture in that folder. Step One is complete.
Step Two involves control-clicking on the Aperture application icon in the foler, choosing Show Package Contents from the pop-up, then going to the Contents -> MacOS folder. Now back up the Aperture executable. Open the executable in your favourite editor (I used emacs). Search for the string 5200. You should see NVIDIA GeForce FX 5200 Ultra OpenGL Engine followed by NVIDIA NV34MAP OpenGL Engine. Replace every single charcter, including the spaces in the string, in the above strings with 1. Save the file, close the editor. And that should be Step Two done.
Now you should be able to run Aperture without getting any complaints. It may crash, delete all your photograps and your 12" PowerBook might explode.. You've been warned.
Please let me know if you had any luck with the above procedure. It works almost flawlessly for me. I've written up my experiences with Aperture so far, which is also where this hack was originally posted. I'll post any updates there.

