I was going to suggest using MobileMe's backup but:
Unlike the previous version of MobileMe Calendar, with the new MobileMe Calendar, the master copy of your calendar data exists in the cloud (on the MobileMe severs). Various calendar clients (such as iCal on Mac OS X and the Calendar app on iOS devices) view a locally-cached copy of the data that is updated whenever a change is made on the server.
So it's a bit tricky! If you comfortable setting AppleScript or Automator to download a file AND you're comfortable making your calendar publicly accessible (assuming someone finds out your public url), you could do this:
Backing up from me.com/calendar
Go to me.com.
Publicly share a calendar.
Copy the URL (web address) from that shared calendar and paste it to the address field in you browser. Do not press Enter or Return yet.
As shown in the example below, change "webcal" to "http":
Example URL from shared calendar:
http://www.me.com/ca/sharesubscribe/1.276... From MobileMe Help
and use that url as your regular download source. I would use Automator and Get Specified URLS and then Download URLS Actions as a starting point.