VMware fusion 4 won't launch due to out of date, running guests
You can try with this command from a Terminal:

VMware 3.x

/Library/Application\ Support/VMware\ Fusion/vmrun list 

VMware 4.x

/Applications/VMware\ Fusion.app/Contents/Library/vmrun list

to see if you have any VMs runnig - if you do, then run this command:

VMware 3.x

/Library/Application\ Support/VMware\ Fusion/vmrun -T fusion stop "/path/to/.vmx" hard 

VMware 4.x

/Applications/VMware\ Fusion.app/Contents/Library/vmrun -T fusion stop "/path/to/.vmx" hard

where you replace "/path/to/.vmx" with your path to the VM still running.