Several bloggers this week have reported seeing this warning.
This symptom may affect authenticated access to Blogger, Google, Picasa, and other services.
In one case (only one case) the blogger in question checked the clock on his computer, and found it to be several months off. In other cases, restarting the browser produced relief, and the symptom didn't return.
It's possible that this originated with the January 2008 Microsoft Security Updates, though we have no verification of that suspicion yet.
No noticeable complaints have been observed elsewhere, outside the Blogger general support community.