I set up a Google calendar last year during 23 Things. I use it everyday and have an iCalendar gadget on my iGoogle homepage. I had already included the US Holidays and Weather Calendars into my own calendar. Before looking at this "Thing," I thought I knew quite a bit about Google calendars.
In reading about Google calendar in this "Thing," I was disappointed to find that you need a g-mail account in order to use Twittercal. I thought that would be a useful tool. I have a Google account, but do not use g-mail, so I won't be able to add items to my Google calendar from Twitter.
I was successful in setting up a calendar for the Minnesota Mineral Club and adding it to the wiki I set up for the Club's field trips. Tonight, I'm going to talk to the Club's webmaster to see if he wants to include the Google calendar on the Club's website. I like that I will only have to update the Google calendar and it will automatically update on the wiki and website. I did struggle a little with getting the calendar into the pbwiki. Instead of inserting a Google gadget plug-in, I was only successful in adding the calendar by selecting the "Source" button and pasting the HTML code onto the wiki page. Other than that, the process was pretty smooth. Let me know if you want to try embedding a Google calendar into a wiki or web-site, I would be happy to try and help!