Of course, there's always a workaround. The following instructions apply to the retail version, and should let you install Aperture on any type of Mac, not just the 12" PowerBook. I doubt Apple will provide support in this case, though. I'll leave it to you to figure out how to do the same for the downloadable trial.
Here's how I installed the retail version on my 12" PowerBook:
- Insert the DVD.
- Open Terminal.app, and type at the prompt:
You should see Finder pop-up with the contents of /tmp/aperture$ mkdir -p /tmp/aperture $ cp -r /Volumes/Aperture/{Aperture.mpkg,Packages} /tmp/aperture/ $ open /tmp/aperture - Control-click on ApertureTrial.mpkg, select Show Package Contents from the pop-up menu, and navigate into the Contents folder.
- Open Aperture.dist using TextEdit.app.
- Search for this line:
Insert a pair of <!-- and --> (HTML comment markers) in the appropriate places:<installation-check script='installationCheck()'/><!--<installation-check script='installationCheck()'/>--> - Save the file and close the editor.
- Go back to /tmp/aperture in the Finder, and double-click on Aperture.mpkg. It should install just fine.
[robg adds: This hint is based on this blog post by the hint submitter. I have modified the formatting somewhat, and added some clarifying details to the pre-instructional text of the hint. I have not tested this one (but would be curious to know how well Aperture 2 runs on a 12" PowerBook).]

