VMWare Fusion 3: vmmon.kext was installed improperly
Here's a tip if you're having trouble installing/running VMWare Fusion 3 on Mac OS X 10.6 Snow Leopard.
The installer displayed this error message:
System extension cannot be used
The system extension "/Library/Application Support/VMWare Fusion/kexts/vmmon.kext" was installed improperly and cannot be used. Please try reinstalling it, or contact the product's vendor for an update.
Turns out I still had remnants of VMWare Fusion 2 installed. I ran the Uninstaller that came with VMWare Fusion 3, then ran the installer again... the install finally worked!