I think it has been suggested sometimes now, but as I am too lazy to search, someone else will have to dig out the old threads.
Having said that, I agree.
I guess it's not that hard to do either, depending on how in depth you'll want to go. Basically, you'd need one "dirt" texture layer that becomes gradually visible.
If Scawen's feeling like a bit of additional work, he could even add a different layer for every underground, like grass or mud.
The only problem would be that it would further increase the graphical load, as every carr would at least need one extra full size texture.
I guess that could be prevented with shaders though, allthough I don't know how much the engine would have to be modified for that.