Then there's the question, which someone alluded to, of how to distribute new credentials to a browser that hasn't logged on before, and how to make sure they're deleted securely after I leave. Do you store the private keys encrypted and send them to the browser so they can be decrypted with my passphrase? I probably don't trust you to store even an encrypted version of my private keys, but I guess I can use a throwaway key for your service.
What I'm most worried about is the question: who is doing the encrypting? Are people sending mail to this address required to encrypt email to me? If not, are you going to encrypt my email as soon as it hits your servers? Not good enough, you may as well leave it in plaintext and use it to serve me ads.
The biggest problem with encrypted email today is that nobody uses it, and this doesn't fix that. It makes it easier (if it works, and I'm still not convinced it does) for people who already use it to do so, but it doesn't convince people that don't use the service (which is going to be a vast percentage of the people that don't use pgp now) that they should want encrypted mail.