UPDATE @ 9:10 AM PST Monday 15 Nov 2010:
The problem should be resolved, as of 9AM PST.
The root cause turned out to be a router failure at Splunk Answers' hoster, which prevented outgoing communication from Splunk Answers to the outside world, but didn't affect incoming connections. Because OpenID is reliant on outgoing connections, it failed.
Splunk Answers' vendor assures us that the problem is how resolved, but if you continue to see problems, please comment on this answer and we'll investigate.
We're having a temporary problem with the hosted solution which powers Splunk Answers. No new logins are being accepted, meaning non-logged-in users can't post, vote, or comment.
The problem started on Sunday afternoon (Pacific Time, US) and we expect it to be fixed soon, hopefully early on Monday.
If you're currently logged in, don't log out!
We apologize for the inconvenience.
We'll update this answer when we know more.
The Splunk Answers Team