Heres a little example of what you can do with Penguinboy's displacement generator, Twister:
The road you see consists of two 45 degree displacement curve segments (created with Twister) slightly above a standard displacement mesh (created normally in Hammer). By using a blend texture for the road (in this case de_tides/blendgrassstonepath) and a standard grass texture for the underlying mesh, one can achieve a fairly good seam between the two. The edges of the road are lowered to form a seam with the displacement below. The curves are possible thanks to the displacement generator, which saves unfathomable amounts of time.
Here's a link to Penguinboy's original post:
http://twhl.co.za/forums.php?pgt=2&action=viewthread&id=13008&pg=1