Looks like a subset of Google Calendar users are running into the same issue when trying to load the service this morning. Based on a flurry of complaints vented on Twitter, it looks like the service gets caught in an infinite loop and keeps reloading / redirecting for some users, regardless of which browser they use.
Update: Google has updated its Apps Status Dashboard and released a brief notice:
We’re aware of a problem with Google Calendar affecting a significant subset of users. The affected users are unable to access Google Calendar, but we’ve provided a workaround below. Google Apps Premier Edition users are not affected by this problem. We will provide an update by June 9, 2010 9:02:00 AM UTC-7 detailing when we expect to resolve the problem. Please note that this resolution time is an estimate and may change.
To resolve this issue, clear your cookies or sign out and then sign back into your Google account.
UPDATE 2: Google has resolved the issue.