I wonder what percentage of parked domains have PPC ads?
Probably the majority. And many most probably using G's own AdSense for Domains [google.com]. You have to keep in mind that when Google bought DomainPark to have someplace to serve its ad inventory, AdSense for Content did not yet exist. Us publishers are the latecomers to the dinner table. Google's always believed in direct nav as part of valid user search behaviour and always will.