As Tony stumbled across a yesterday, Google is getting very close to launching their public calendar search tool. The still unpublished Google Calendar "What's New" page has more details about the feature.
Now you can search the web for events and add them directly to your Google Calendar. Within Google Calendar, type an event search into the search box like "Symphony in San Francisco" and click "Search Public Events". You'll see a list of events that you can easily add to your Google Calendar.