Instead of doing a 1-to-1 account mapping with an identity provider, it might be a better idea to keep your own system accounts, and map OpenIDs to your accounts. Your users have an account on your system, but uses OpenID as the authentication mechanism (instead of user/pass).
For example, I might have multiple OpenIDs that I can log into StackOverflow with.
The advantages are:
- Does not restrict your users to only one identity provider. Even if Google goes out of business tomorrow, your system still works.
- You can handle login/logout of your system separately from Google.
- OpenID is meant to work this way.