Google launched a mobile version for Google Calendar, available at google.com/calendar/m. To use this application, you need a mobile browser that supports XHTML, cookies and SSL (for authentication).
The features are limited to browsing the list of previous and upcoming events and adding new events using Google's natural language processor. Even if it's very basic, the mobile Google Calendar shows a link to the map of the place where the event happens, so you know how to get there. You can also select the calendars used by Google to display events.
For notifications, don't forget to register your phone from the desktop interface and to set SMS reminders for each calendar important to you.