Script-Fu -> Photo -> Highlights to Sky
Over-exposed?
No Problem.
This is a an adjustable script,
that creates a Sky and Clouds in an image highlights.
So if you have a "blasted-out" sky, this script can help.
Script-FU > Photo > Highlight to Sky and Clouds
Original Author's site:
http://www.farcrydesign.com/GIMP/BlueSkyClouds.html