Hi zzu, this is very simple, but must to work fine. It is the same principle that the clouds, take a image map, and break up with some procedurals details. you can play with many things, like the color, the scale of the details, etc.
For others interested users, I am actually working in a "dynamic" city lights shader. as Bigben suggested sometime, the idea was create a anti-sun vector, relative to the planet position. of course, nothing of that will be necesary if the clouds can be rotated with the planet. (another of my wish for TG4)