10.5 System Upgrade - another blue screen cause
Not so long ago I upgraded a 12" PowerBook from Tiger to Leopard OS X, resulting in a blue screen on restart right after installation.
The cause was the Unsanity Application Enhancer installed on the old system. If you do an upgrade, check if you have Application Enhancer (a helper Extension in System Preferences, used for Applications like Airfoil for example) installed and uninstall it if its there.
In case the blue screen is already there, help is provided on the Apple's Support Discussions:
http://discussions.apple.com/thread.jspa?threadID=1197402&tstart=45
Boot your blue screen Mac in Firewire-mode if possible (by pressing T on startup), connect to another computer and remove the following files:
/Library/Preference Panes/Application Enhancer.prefpane
/Library/Frameworks/Application Enhancer.framework
/System/Library/SystemConfiguration/Application Enhancer.bundle
/Library/Preferences/com.unsanity.ape.plist
If Firewire-mode doesn't work you may have to remove your hard disk and place it in an external drive to sort t out. For me Firewire-mode worked fine though.