A Google Summer of Code participant has been selected to create for blender a Sky rendering system. Most of the information can be found here:
Blender Wiki
The sky will be fully animateable, complete with fully morphable clouds so that they can be a specific shape. To be put bluntly,
this thing is going to rock. The sky renderer will even simulate pollution, fog, and the like, and
more than one sun may be possible.
But the main thing for me will be the clouds. The Sky wiki page links to this, possibly because it uses the same/similar algorithm:
http://www.ofb.net/~eggplant/clouds/
If the clouds look anywhere near as good as the ones in that page, then I will be extatic. They look absolutely amazing. This new sky thing will bring Blender closer to a replacement for Terragen, as far as creating skyboxes goes.
Awesome stuff!