This article is based on the steps outlined in MacOSXHints.com's article on bypassing installer checks.
I started with a newly formatted HFS+ volume upon which I would install Leopard (10.5). Boot off your installation CD by holding down the "C" key. Once it has started, click "OK" when told "Mac OS X cannot be installed on this computer".
Under the Utilities menu, select System Info to open the profiler. Make sure the left pane has "Hardware" selected. The right pane will show the model name and the model identifier. Make a note of this identifier (on my machine, this was "MacBookPro2,2"). Close (quit) the profiler.
Under the Utilities menu, select Terminal.
Create a temporary directory on your newly formatted volume. This will be the staging area for the modified install.
$ mkdir -p /Volumes/Untitled\ 1/staging/extracted
Copy the installation package directory (and it's contents) to the staging directory.
$ cp -r /System/Installation/Packages /Volumes/Untitled\ 1/staging/
Extract the installation package.
$ cd /Volumes/Untitled\ 1/staging/extracted
$ xar -x -f ../Packages/OSInstall.mpkg
Determine the expected hardware versions for your version of the installer.
$ grep hwbeSupportedMachines Distribution
You will now see which identifier this CD was intended for. In this case, the version would be "MacBookPro4,1". Replace this with the string you found through the system profiler.
$ sed -i.bak 's/MacBookPro4,1/MacBookPro2,2/g' Distribution
Repackage the install package.
$ xar -c -f ../../temp/Packages/OSInstall.mpkg *
Start the installer.
$ /System/Installation/CDIS/Mac\ OS\ X\ Installer.app/Contents/MacOS/Mac\ OS\ X\ Installer \
../../temp/Packages/OSInstall.mpkg
