I made this model with the plugin Rich suggested:
http://sketchup.google.com/3dwarehouse/details?mid=bc92eca8611ed04d2470944666440e50
Then just out of curiosity, I simply reloaded the Earth image with a Mars image and it kept the spherical UV mapping perfectly:
http://sketchup.google.com/3dwarehouse/details?mid=6ed69ca0de0718c2bb014d52f42fdd7f
so you can even use either model any reload the image in them.
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