Sync took a different tack, and started off with “what if we didn’t want the data? What if even having that data was a failure state?” That led us to cryptography. Sync uses strong crypto to encode your data before it is uploaded. The secret phrase is the key to this encryption, and we never send that anywhere to keep your data secure. This really means that Mozilla can’t see your data, giving you full control. (Which is great, because we really don’t want it!)
via Mozilla Labs – Sync in Firefox 4 Beta. I love systems that take this sort of approach to data, we don’t know it and we can’t know it.