If you can't start Parallels after Lion upgrade do this:
Open Terminal and past this 5 command lines one after another. After pasting first line, sudo command will ask you for your user password, so you have to enter it and press Enter (only after first command):


-------------------------------------------------------------
sudo kextutil "/Library/Parallels/Parallels Service.app/Contents/Kexts/10.6/prl_hypervisor.kext"

sudo kextutil "/Library/Parallels/Parallels Service.app/Contents/Kexts/10.6/prl_hid_hook.kext"

sudo kextutil "/Library/Parallels/Parallels Service.app/Contents/Kexts/10.6/prl_usb_connect.kext"

sudo kextutil "/Library/Parallels/Parallels Service.app/Contents/Kexts/10.6/prl_netbridge.kext"

sudo kextutil "/Library/Parallels/Parallels Service.app/Contents/Kexts/10.6/prl_vnic.kext"