There was a interesting comment on a MacOSXHints entry about Filevault that I found interesting (check out the one by wastemytime).

While early versions of FileVault may have been susceptible to the corruption you fear, you should be aware that Apple quietly introduced a new version of it in Mac OS X 10.4.7 last year.
In earlier versions, the sparseimage's headers were written at the end of the file (they should've been called trailers then), so, if there was a problem writing or compressing the image, it could easily be corrupted.
With the 10.4.7 update, they introduced a new version of sparse disk images so the headers are at the beginning (appropriately enough), making them much more resistant to failure.
The catch is, if you created your FileVault with an earlier version, it continues to use the old, more fragile format. The solution is to just turn FileVault off, then turn it on again (probably using the secure erase option). The new FileVault will use the new format.