For a camera manufacturer with a significant "film" user base, in-lens IS is the only option available. An in-body IS is not an option at all, for obvious reasons, since if the lenses don't have built-in IS, they will not provide I.S on the film bodies.
I'd be surprised if backward compatibility with film SLR were still high on the priority list of Canon and Nikon...
I think that Canon/Nikon not having released body side VR bodies yet does have more to do with other factors:
- past investements in lens VR technology
- concerns regarding reliability and basic body performance with body side VR (difficulty of maintaining the sensor exactly perpendicular to lens axis, impact of shocks when a body falls,...)
- advantages of lens stabilisation
-> the optical viewfinder is stabilized too
-> technology enhancement can be rolled out lens by lens and therefore tuned for each lens's specific requirements that are going to impact typical vibration patterns (weight, inertia center location, typical handling,...)
-> location of the acceleration sensor probably makes more sense in the lens than in the body,
- lack of combined solution using both lens and body side VR because of lack of communication between body and lens as far as VR real time information goes