The nebula is very well done, however the lunarcell moon doesn't fit in with the rest of the image.
Radial blur on the lensflare as mentioned here before would help make it fit in more with the image.
Using lunarcell for planet clouds and then adding some hand-painted touchups is a good trick.
I've not been too satisfied with any hand-done earthlike planets yet as they look too painted personally, while you can get a planet without water to look pretty good.
Did I say I liked the nebula? Could you do a 1280x1024 sized version of that nebula with no planets, moons or fancy text? I'm still trying to figure out a good way of doing my own nebulas. I've just not been having luck with larger scaled stars.