Calendar backup while away from time machine
If you are running Lion, iCloud, launching in October 2011, will soon provide cloud-based back up for calendar information.

To manually backup iCal entries, Apple recommends that you either


Save an iCal Archive on your hard drive (File » Export » iCal Archive...)
or
Export an individual calendar to a file on your hard drive (use this method for any CalDAV calendars) (File » Export » Export...)


To protect against hard drive failure, copy the resulting files onto external media.

If you wanted to sidestep the Apple recommended process, and simply back up a single folder, it appears your calendar data files are stored in ~/Library/Calendars

Additionally, Google provides instructions for how to sync iCal with Google Calendar.