Ok, got it.
Isn't that already essentially addressed on the mobile client with PIN support?
Now if we're talking about bringing that over to the desktop side, it sounds appealing, except that somewhere near half of the users I've read posting requests in that area know that the data can still be read on the back end, either in the same account looking at the Evernote folder and sqlite database itself, or the same from another admin level account on the same computer.
Adding local per note/notebook side encryption then serves both halves of the feature request. It blocks both in-evernote and out-of-evernote read attempts.
And this is completely separate from any discussion of the evernote server side handling/non-handling/awareness of the encryption. This would be local client encryption only, with data decrypted on every authenticated read (be that the Evernote user, or a sync operation).