Thanks for the tips!
I've done some work in the last few days according to RobA's marvelous tutorial, and have come up with this:
world_map_progress.png
Obviously, I don't plan to leave everything as grass (as you can see with my desert attempt, which I currently don't care a whole lot for).
I've been playing with the polar ice caps and have come up with some stuff that's ehh... decent, but not really convincing. Gaussian blur is only getting me so far. I'd kinda like to go for a Northern Canada look, a la this satellite photo:
http://www.debian.org/devel/developers.map.jpeg
Neither my northernmost nor southernmost continents are entirely covered with ice, so I need a good transition and a simple Gaussian blur isn't cutting it. Any thoughts would be welcome, and it doesn't have to be exactly like northern Canada.
Actually, any suggestions at all would be welcome. I need to do deserts, forests and mountains and at this scale, things get a little more complicated... for me, anyway.