The problem appears to relate to some new terms of service, to which users must assent in order to use the site. Unfortunately, the overlay sometimes keeps appearing even after the terms are accepted, blocking access to the page. We were able to get back to the site by declining them, though we're not sure if the new features were implemented in that case. It's also possible mail client users won't even notice the problem. Yahoo experienced an unrelated outage a few months ago, and other major services like Google Talk went down last week.

Thanks, Anonymous!

Update: Yahoo has given us the following statement on the problem:

Yahoo Mail, Messenger and other areas of Yahoo may currently be inaccessible or slow to respond to some users in certain locations. We are working to correct the issue and restore all functionality immediately. We know that this may have caused some inconvenience and we apologize to our users who might be affected.

Update 2: Yahoo representatives have issued a statement that all services have been restored:

Yahoo Mail and other services are restored. We know that this may have caused some inconvenience and we apologize to our users who were affected.