TwitterCal is a new application that lets you update your Google Calendar directly from Twitter.
Add the TwitterCal bot as your friend on Twitter and send a direct message in order for your Google Calendar to be updated. Be sure to include the "g cal" directive when creating a message for this Twitter bot. Similar to other bots, including 30Boxes and Menuism, you'll need particular key words to be written at the beginning of your message for it to be properly recognized and processed. To sign up, you'll only need your Twitter username and your Google email address. The TwitterCal application has added automatic "friending" for when you sign up for the service, which is supposed to speed along the process, but it doesn't seem to have kicked in yet.
As with other automated services that integrate with other applications, it may take a while for the appointment to actually show up on your Google Calendar. Nevertheless, it's helpful to have a level of SMS access to your Google calendar when you're away from your computer.