Tony Hirst found a simple way to place the events from a Google Calendar on a map. Assuming that the calendar is public, all you need to do is to find the XML address of the calendar, enter the address in a Yahoo Pipe that extracts the locations and generates a geoRSS feed, click on "More options" and copy the link to the KML output. You can enter the URL in a Google Maps search box and you'll see the events on a map. The items can be saved to My Maps or embedded into a web page if you select "Link to this page".
Tony notes that "the call to the Yahoo Pipe sometimes times out in the map. To guarantee the map displaying the KML feed, you could always save the KML feed as a KML file, then upload it somewhere and use that URL in the Google Map search box".