This issue has been resolved.
We're aware of a problem with Gmail affecting a significant subset of users. The affected users are able to access Gmail, but may experience delays in sending or receiving messages.
Google engineers are currently working to mitigate this issue and will restore normal service as soon as possible.
No workaround is available at this time.
You can access Google's status dashboard at www.google.com/appsstatus for more detailed information about this outage.