These are screen shots from a flight sim I am working on.
We want really high detail low down flying, without sacrificing Visible distance, so came up with this technique, which draws a textured ground plane, and then alpha fogs the close in terrain into this.
This is created using the Power Render Landscape Engine (www.egerter.com), with a few extra goodies I have added.
The Ground plane does not have to be flat, and can actually be a simplified mesh of your entire terrain area. Flat is more suitable for my uses tho.