I did some more digging. Apparently, if the contractor had followed Wright’s plans to the letter, the structure wouldn’t have even supported its own weight. The builder doubled the support on his own, (Wright was furious at having been shown up) which prevented immediate failure at the time of construction, but which still hasn’t proved sufficient.
The propping up of the cantalevers alone cost about a million and a half; that was only part of the work that has been needed. Total work ran about $11.5 million for the latest round.
Some details can be found here (also mentions a leaky roof on another Wright project in passing): http://www.structural.net/News/Media_coverage/media_fallingwater_toh.html
Wow...interesting! Wright comes off in that piece as a bit of a megalomaniac.