Currently, if one is logged out and attempts to commit to a proposal in area51 with a registered email address, an error appears that the email is already registered. There should be some guidance to let the user know they need to log in to register in this case - since you have the email already, you could even simply immediately trigger an openid login using their registered openid provider. Or simply show a link to, or redirect to the login page.
This is a good idea, so we implemented it, more or less:
When following or committing with an already-registered email, we ask you to log in if you already have an account and the email is yours. After logging in, you are returned to the proposal page with the follow/commit popup open and the form filled out.