Sorry to double-post, but I just found this:
Is my Cloudified data secure?
Yes, only you have access to your data. Bitcasa encrypts your data on your computer before uploading to the Bitcasa system. Nobody at Bitcasa can see your data or filenames.
If that's true, then they can't de-duplicate data across customers. And they do what SpiderOak does. Or at least that's what I think it means.