Re: having an object depresss geometery as it rolls on it...

So a softbody solution would involve making the surface a softbody, making the particles collide (zero friction zero resilience) with the rolling object, and then just moving it around. If the collision solution introduces too much distortion, instead you can parent a field to the rolling object and have it force the particles straight down.

The displacement map solution would involve creating an animated black-and-white sequence showing the trail growing from the initial dent to the final trough, then applying that as an animated displacement map and syncing it with the object's motion.

Personally, I think the softbody solution is probably less hassle--it stays more flexible, too.

-Steve