Some good pointers there, folks, thanks. I'll certainly have a look at the Middle Earth project, wasn't aware of that.
Cyphyr – yes, I have followed that approach, but it has a few drawbacks too. I'm creating these worlds for a Sci Fi setting, so having the map on a sphere (perhaps to create a spinning globe animation) is a requirement as well as creating planar map projections. While it's possible to create realistic terrain by hand in, say, photoshop, it can be tedious without procedural assistance of some sort.
Maybe you're right though, I should start with a plane, in World Machine or similar, and then wrap the result around a sphere. I understand that if the flat map has a 2:1 aspect ratio then it is possible to wrap it around a sphere in a 3D program without too much trouble. That seems to be the approach taken over at Natural Earth III (
http://www.shadedrelief.com/natural3/index.html) which I stumbled across last night, and that looks stunning.
Thanks for the advice, guys, most helpful.