Looks like Google Calendar is down, and not just for me, even downforeveryoneorjustme.com says so.
By the way, if you visit http://calendar.google.com you will see the following message as shown in the screenshot below.
Thank God I had my Calendar synched to an Exchange server or else I would have missed one of my appointments. Is Google Calendar down for you too? The Google Apps status does not show anything about the outage yet.
Update: Looks like it just sprang back to life and is back up again. Did you face any issues?