In the "come on you wanna use our service" information that these cloud storage solutions provide, you will see words like "secure" "encrypted" and many other synonyms for "your data is safe and secure with us".

The good thing about cloud storage is your (potentially) confidential information is only viewable to you, anyone you share it with, several thousand trusted employees of the storage provider you choose to use, and possibly the law enforcement officers in whatever jurisdiction that the service is hosted in.

With all the talk of cyber-crime and foreign governments gaining access to internet accessible systems. It would pay to give some thought to making sure the commercially sensitive data you have hosted online is genuinely viewable by only those you wish.

I've found that there are products that enable the use of Client Side Encryption. Meaning you create a folder on the cloud store using the CSE client and then everything you upload through the client is encrypted by yourself.

I have just installed Boxcryptor on my Android device and it works well. The beauty of it is that it uses encryption compatible with Linux encFS.

I found that if I create a Boxcryptor folder on Dropbox, using my Android phone, and then Dropbox does it's thing and mirrors it down to my Fedora workstation. I can use the nice graphical utility Cryptkeeper to import the encFS folder and mount it somewhere in my home drive and have full access to the documents.

So for the paranoid or those who genuinely need to guarantee data security, implementing CSE will give you more assurance that your cloud storage administrators aren't grabbing your data and using it for their own purposes.

Refs: 

https://forums.dropbox.com/topic.php?id=48652

http://www.makeuseof.com/tag/5-ways-to-securely-encrypt-your-files-in-the-cloud/

https://www.boxcryptor.com/

http://blog.boxcryptor.com/how-to-use-boxcryptor-with-encfs-in-ubuntu-ma