To follow this tutorial you must be running Mac OS X Leopard 10.5.3.
Launch Address Book by selecting it from your dock
Select Preferences from the Address Book Menu.
From the General Tab check the Synchronize with Google box. Then click the Google... button if a popup window does not appear.
A popup window will appear asking you to allow Google to access your contacts. Click the Agree button to continue.
You will be asked to enter your Google Account ID and Password. Do this then click the OK button.
Connect your iPhone to the computer then open iTunes by selecting its icon from the dock.
Make sure your iPhone is selected from the list of devices on the left.
Click to select the Info tab. Then check the Sync Address Book Contacts box.
Now click the Sync button at the bottom right of the screen.
Your iPhone will then sync its contacts to Address Book as normal. However, it will at the same time sync your contacts to Google Contacts!