Update 9/24/13:
As of 10PM last night, Google updated their Google Apps Status Page to indicate that the issues were fully resolved.
Original Post:
Google has reported service issues on its Google Apps Status Page today. These issues may affect Gmail, Documents, and Presentations and may result in significant email delays. As of 3PM, Google hoped for resolution of the issues within an hour. Our email administrators caution that even once the problem is corrected, it may take some time for backed up emails to deliver.