What are the alternatives for storing client side encrypted data?
Personally I would not trust any JavaScript or Flash provided by operator to safely hide my data from him. How can I know if data is really encrypted on my side, or is it just a smoke and mirrors? Probably that's the reason modules like that was never popular - they simply can't be trustworthy, so why to bother developing them if hardly anyone will trust them?

And if user is technological enough to know if data was encrypted on his side or not, he would prefer the other way. There is a way that allows user to be sure their data was encrypted on their side:


Make user install FireGPG plugin, or something analogous for his browser, to be able to encrypt any text field with his tool, that's under his exclusive control, and developed by Open Source teams not connected to you.
On Drupal side, Field Validation should stop Drupla from saving data that does not look like PGP block. Clientside Validation should stop user from posting it in the first place, but you never know for sure with client-side mechanisms.
Secure Pages should mitigate the risk of accidental access to data if user will manage to send unencrypted despite clientside validation.