I'm getting an error trying to login to Stack Overflow and Meta Stack Overflow with my Google Apps account (I remoted into my home computer to post this). The message I get is "Message signature was incorrect".
Google appears to be suffering some sort of outage, it's not just a Stack Exchange problem.
As a work around, if you register a new account at openid.stackexchange.com with the same email address that is associated with your Google account you'll be able to login. We auto-magically map up users with the same verified email address basically. Login in via Facebook will also work, for similar reasons.
Note that the email must be exactly the same, so don't use any
Note: this seems to be intermittent or random, as many Google OpenID users are still able to login.