Yes, I do that for certain files in my Dropbox. But opening one of these files requires opening truecrypt, mounting the file and then opening the files inside the mounted drive. I wish that I could have this security, but be able to open an encrypted file in one-click. This could be possible if I were able set my own private key, and then the Dropbox (or Google Drive) client would encrypt all of my data as they transfer it to their servers.