This is one I've been meaning to share for literally months; I can now finally close the tab I've been keeping it open in.
Uploaded by BigfottStudios.
The blurb reads:
This video shows how Fred Astaire's famous "dance around the room" was filmed by Stanley Donen in the 1951 movie "Royal Wedding" (now in public domain). To see a detailed account of how this video was made, visit http://www.bigfott.com/Astaire_Unwound.html
I definitely recommend following the link and reading the detailed account, particularly the way the effect was recreated for Lionel Richie's "Dancing on the Ceiling" video, it really is something. I imagine, also, that a similar technique was used for filming some of the dream-within-a-dream scenes in Inception....?