Play with lights/ambient to make it look cool, as oposed to making it phisically acurate another problem is the specularity, you need to make a map: no specular on continents just a little bit on sea. also the atmosphere isnt specular turn to 0 all specular shading there
Also download a cool clouds map (for example from the bluemarble project)and add an extra sphere with the inverse of the color as the transparency and bump add another layer with procedurals if you wish.
Also the atmosphere is thicker near the ground... i havnt been able to make the damn volume shaders work properly but ragecgi maybe knows how to fake it in other way so now i think about it:
Rage how to fake the atmosphere going thicker towards the ground on the edges?