I think that Norton Antivirus and to a better extent Norton Internet Security should integrate with virtual machines to protect the host system and other systems on the network that may inadvertently interface with it from viruses and other threats it might pick up.
I use my virtual machines for legacy programs not compatible with Windows 7. I leave it unpatched and unprotected because there is really no point running and keeping up to date two separate security applications, let alone two lots of windows update downloads every patch tuesday.
There is still a concern about threats targeting the windows xp virtual machine, especially since there is a strong connection between the guest and host systems with XP Mode. So instead of running two separate security applications I'd like to run Norton Internet Security on the host and have the peace of mind knowing it was integrating with the virtual machine and keeping my host protected, as well as other PC's on the network, from the virtual machine.