Google Earth and Nasa World Wind are great sources of inspiration,(there's spot in North Africa I keep finding and wanting to do) neither of them has particularly good resoultion for height maps. Nasa's format of .hgt files might be convertable.
My first map Death Valley did use a greyscale height map I found. I added a little more detail to it by using the satalite photography of the same area.
You will get more res from an OS map, 'though it does take ages to convert one by hand. My efforts here -
http://taspring.clan-sy.com/phpbb/viewt ... 9515#49515
One of things I like about the currnet format is that there are many ways to make a map, which gives us a good variety of styles. When someone else makes a good map, I push myself to maintain the standard, and to try to find my own way improve.