One of the nice new features of the iPhone 3.0 software is the ability to add a custom CalDAV account. Since Google Calendar supports CalDAV, this means that you can now add your Google Calendar account to your iPhone! And not only are you able to view calendar entries, but you can add, delete, and modify them too!
To get started, navigate to to the following screen by choosing Settings -> Mail, Contacts, Calendars -> Add Account... -> Other:
Now choose Add CalDAV Account and enter your Google Calendar account login details. The server should be set to www.google.com. When you're done, press Next and wait for the iPhone to verify the information:
When it finishes, go back to your home screen and open your calendar. Press the Calendars button at the top left to view a list of all available calendars. You will now see an All Google option listed:
If you only want to view your Google calendars, you can select the All Google option. Otherwise, choose All Calendars to view all of them. Now when you add, delete, or modify an entry on any of your Google calendars from your iPhone, they will automatically be synced with Google!