Jim's work to bring proper key encryption to MMultiBit/bitcoinj is going to be awesome. Until that work is complete, I've considered implementing a more heavy-handed interim approach: Require a passphrase at application start, then use a symmetric cipher (AES) to simply encrypt the wallet file stream.

Would this be at all a worthwhile endeavor, or just wait for Jim's more featureful approach?

Clarification: I could do this in application logic, and/or bitcoinj. I've investigated bitcoinj, and would need a slight augmentation to also use wallet auto-save (pass in a stream factory along with the file name).