took like 15 minutes and i want to improve.
instead of making it 1 sphere, you could try making it two spheres, one for water and one for land.
Also, you might want to put an atmosphere around it too
where the water touches the land, make that a sandy yellow colour to show beaches.
make the land less blue, and turn down the spec of the land.
hehe I was going to do a quick tut on this but I sorta put it off XD
Here is a planet I put together a while ago…
http://blenderartists.org/forum/showthread.php?t=149619&highlight=planet