UPDATE: Google has reported that service has been restored:
The problem with Google Calendar should be resolved. We apologize for the inconvenience and thank you for your patience and continued support. Please rest assured that system reliability is a top priority at Google, and we are making continuous improvements to make our systems better.
UPDATE: Google has updated us with the following:
Google Calendar service has already been restored for some users, and we expect a resolution for all users within the next 1 hours. Please note this time frame is an estimate and may change.
We're investigating reports of an issue with Google Calendar. We will provide more information shortly.
Users of Google Calendar see 'Server Error' page intermittently.
The PA Virtual Tech Department will will update as we recieve additional information. You can see full details at Google's own status page: http://www.google.com/appsstatus