The answer is, of course, to store your data online and encrypt it.
There remains the space for a cloud sync service that encrypts client side and provides good enough clients for every major platform.
Spider Oak comes close, but it's just too damn ugly an interface and isn't the "Install and forget" option that Dropbox is.
If I understand correctly, it's not as secure/confidential as Spider Oak: the encryption key for file A is Hash(A) and your own key is only used to gain access to Hash(A) in order to decrypt the files. This lets them deduplicate more efficiently on their end, but it also means they can determine if two users have the same file. It also has some other repercussions (there is a HN story about it). It's still a lot better than DropBox, though.
However, I also know that since their website allows me to access data and reset my password, their key management doesn't prevent Dropbox employees from viewing my stuff.
Dropbox had said that they encrypt data before storing it at Amazon, but their systems see all of your raw data because they do deduplication, and because they could reset your passphrase, and because client-side encryption of stored data would make web access very complicated if not impossible.
If all that weren't enough, the dropbox forums, long before the early 2011 PR problem, had threads about using truecrypt containers on dropbox shares to ensure security. It also had feature requests to add client-side encryption to the dropbox client. If some people didn't get the message that dropbox has access to raw data, after all of that evidence, they have only themselves to blame.