Update: Google has resolved the issue with the Postini service and mail should begin to arrive normally.
Webmail by Google as well as Gmail in general seem to be suffering an outage, involving the Postini service. Google is aware of the problem and is working to fix it as soon as possible. Please check back here for updates, as they become available. For current information on the outage, please visit; http://www.google.com/appsstatus#hl=en&v=status&ts=1393425904990