Today I did search the web on tutorials on how to create nice planet textures and came across the easy to use "LunarCell" plugin for photoshop. Actually you can also use GIMP with the PSPI plugin-in to run the "LunarCell" script (I encountered problems selecting colors but everything else worked nicely).
Here's the result using GIMP 2.6.11, PSPI 1.0.7, and LunarCell 1.8 (evaluation copy) to create all textures and Blender 2.57b for rendering and compositing:
In Blender I used Blenderguru's tutorial for the earth.
In GIMP I created a canvas with width 4096 and heigth 2048.
In "LunarCell" I created the following textures, which were used in Blender as:
mural -> diffusemap 1
map color -> diffusemap2
land bump -> bump map
cities -> night map
clouds bump -> cloud map
ocean mask -> specular map for the ocean (I do not have an ocean in my example)