I forgot to look at this landscape generator earlier, so what does Terragen (http://www.planetside.co.uk/terragen/)have to offer. It seems to specialise in the rendering and procedural/heightfield generation side of things. It uses vegetation models from say xfrog and I am not sure how well it would accomodate a 'design proposal' type usage.
That said - check out these images from their website gallery - super:
[images from the Terragen website]
WoW - just a bit for fun
[journeytime = 1.5 hours]