I just keep a TrueCrypt volume in Dropbox. I can read it on Mac, Windows, or Linux and it's secure. I use this for taxes and stuff, and don't encrypt non-sensitive stuff so that I get more granular versioning.
With TrueCrypt you can actually unmount it and make it inaccessible even to people who have physical access to the machine. I don't see any instructions on how to do this with SecretSync, even in their FAQ, but from their documentation it appears this would need to be done:
1) Open the app and click Stop sync
2) Delete the SecretSync folder (make sure it's really deleted and isn't in the Trash or Recycle Bin)
