Terrain stencils, basically a shader where you can paint more detailed textures onto a terrain using a greyscale stencil image - if you've ever used UnrealEd or tried stencil layers in blender you'll see what I mean - the great thing is that you can seamlessly merge textures, look at the edge around the water where the grass meets the rock. I'm trying to get 3 layers or more to work but code isn't my thing, I need to work out how to change the Blending between textures for that to work - at the moment it's just texture1.rgb*stencil.rgb stuff, that's about the limit of my maths skills:
If I understand right, you have 3 textures - grass, rock, and the stencil (grayscale).
http://en.wikipedia.org/wiki/Alpha_compositing
If you replace the "alpha" layer with the stencil, then this gives several formulas which could be applied to multiple layers (adding another stencil + image for each layer).
For example,
layer1 * (1 - stencil1) + layer2 * stencil1
->
(layer1 * (1 - stencil1) + layer2 * stencil1) * (1 - stencil2) + layer3 * stencil2
Where the stencil is 1.0 it's entirely showing the "top" layer, where it's 0.0 it is entirely the "bottom" layer, etc. Each layer applied on top basically substitutes all previous layers as "layer1" in the original formula.
I'm not actually sure if a cg shader can be written to take an arbitrary number of images (instead of a fixed number) so you still might need to write separate shaders for each number of layers you wanted to combine.
Anyway, it's a good idea for adding detail without adding polies
if I had the time or knowledge to make a track I'd probably use it.
As to the grass, have you tried making the bottom edge fade away a bit? The sharp bottom edge of the polygon is always what draws my attention, it's less visible with more grass but still kinda there, especially in certain lighting.
No screenshots of my next project yet, to learn how to use Blender I'm making a Mustang (mostly for show, extrapolated it'll have something like 200,000 polies when it's done, but it should look smooth at least). Once I've got a pretty good grasp of how to do stuff I'll try to finish the Aronde.
I've also been pondering nicer blurred wheels, once I have a tech demonstration I'll post it. Have to do it in ZMod because I don't know enough about Blender yet.