I think this is very clever: use Grease Monkey to decrypt content that’s stored on 3rd party servers. In the example the body of the RSS entries are encrypted. Then the user subscribes to the feed via Bloglines. Finally when viewing the entry a Grease Monkey script notices the encrypted text and decrypts it so the user can view it. The same idea would work for email sent to a hotmail or gmail.
I find it interesting that this use case wasn’t a standard part of browser design.