I have worked on a deformer plugin that does this very thing for use with tanks and vehicles as well as human feet.  There is an existing plugin for Carrara 5 from Akuro Digital called the 'potentializer' that can be used to do it (sorry, cant easily find the link for that).  Mine allows for variable size 'feet' that deform a terrain or vertex object and I added a matching shader for the disturbed earth - but that part didnt work on large terrains.  My plugin will work well for close ups if I subdivide a spot area but for large terrains its slow and to add detail like tank track imprints it would need a lot of polygon detail or detail in the path of the vehicle.

The beauty of a plugin like this is that you apply my deformer to the terrain, select which objects are 'feet' then when you animate or render it builds up the changes frame by frame in a copy of the terrain or vertex object.  Change the path of the vehicle and the tracks left behind are reset.

Its still a beta version though and after working on the Jiggle Deformer I've found out how to do it so it works much much quicker and so I need to revise the code. 

The beta works though, I used this on an animation of an excavator you can see in my website gallery.  Look closely and you will see that it leaves a rut behind it.  For digging the hole I made a morph and for scooping and dropping the earth I couldnt use physics and had to do it manually...which sucked.

And I did another test using the ATV from Daz leaving a trail in the sand which is much more clear - it also uses a terrain following plugin I've been working on but havent perfected yet.

Reply to me directly if you are interested in the trying out the current beta for C5-6 (windows only) if this is what you are after.

Jeremy
www.Sparrowhawke3D.com