For terrain: In GPU Gems 2 (the book) there is a chapter talking about a technique called Geometry Clipmaps - I think it is where you perform a texture lookup in the vertex shader and displace the vertices in realtime, but I am not too sure on this one. In the chapter is states that a demo of this technique had a 20 billion sample grid of the US in realtime. I am not sure of the benefits of displacing vertices on the GPU over modelling the terrain, except for maybe "seamless" transitions and less actual modelling.

Clouds: MrCode is right, the artwork will make a good looking cloud. Just as an idea, you could always have close-up clouds as volumetric/particle based clouds and as they get further and further away change to camera-facing billboards, that way you get the appearance of each cloud is volumetric and you can "fly through" them.

Cockpit: Like MrCode said, there is a beta version where you can render panels, etc into bitmaps.

Hope this helps a little,
Adoado