When I'm using someone else's computer to check email? Like I said, I'd never trust my browser to touch my private keyring, so that isn't a question for me. Friends who know approximately nothing of GPG could get a message stating that there is *some* reason to expect that the email I sent is actually from me other than the From header.
This brings up the problem that there needs to be a way to communicate that a signature is expected. Anything in the mail doesn't work, so there needs to be some server-side implementation for this.