Adding background to the landscape and mixing with mist In order to create clouds in the background of landscape I followed instructions of the lesson and used the Image node to add image needed in the result. Then I mixed that image with color (by using AlphaOver node) added by the RGB node to make environment look more realistic. 
So, we have clouds created. 
Then Mist socket from the RenderLayers is used to give infromation to ColorRamp node about the mist we want to get. ColorRamp's Image output socket is connected to Fac socket of the Mix node. Our clouds are added to the first Image socket, and the main Image from RenderLayers - to the next one. 
I would like to ask about this way of mist creating. How is it possible to avoid of translucence of clouds through the island and at the same time leave mist? I tried a lot and didn't found any ways to combine these 2 images without getting problem when clouds can be seen through the front image (in this case, island).

Screenshot of the nodes set up.
              

To point out your attention at the problem, screenshot of the rendered image if ColorRamp node settings are changed (mist managing).


UPD.

I'd like to say that I think my island wasn't behind the clouds but rather the island lost transparency according to the principles mist settings are processed in Blender. You might want to look for references - http://wiki.blender.org/index.php/Doc:2.6/Manual/World/Mist#Transparency,  this one - http://en.wikibooks.org/wiki/Blender_3D:_Noob_to_Pro/Mist_-_Make_Objects_Opaque. I'd like to use the last one as a manual but I was unable to reach everything I wanted.
The problem is that your clouds should not be in front of the island, but behind!

EDIT: (after messing with this for a while...)
You need one more mix node.
In other words you need one to key the clouds behind the island.
And another one mix node to mix the mist over the island and the background.





I disabled mist again, according with your comment.
The hardest part is controlling how the horizon meets the sky since the mist pass takes care of the island but not of the ocean in the distance, so I added a mask to affect the alpha channel at the horizon line of the render layers node. Then I used that new mask to control the alpha over that mixes the Island and the sky.

The modified file I placed here:
http://www.pasteall.org/blend/28181

It's been a lot of fun playing with your file!