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REF:
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http://www.google.com/support/mobile/bin/answer.py?answer=138740&topic=14252

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How do I setup Google as Exchange account on my iPhone, iPad, or other iOS device?

–Settings
–Mail, Contacts, Calendars´┐Ż
–Add Account
–Select Microsoft Exchange

–In the Email field, enter your full Google Account email address. If you use an @googlemail.com address, you may see an “Unable to verify certificate” warning when you proceed to the next step.
–Leave the Domain field blank.
–Enter your full Google Account email address as the Username.
–Enter your Google Account password as the Password.
–Tap Next at the top of your screen.
–Choose Cancel if the Unable to Verify Certificate dialog appears.
–When the new Server field appears, enter m.google.com.
–Press Next at the top of your screen again

Select the Google services (Mail, Calendar, and Contacts) you want to sync.


http://www.anysitesupport.com/how-do-i-setup-google-as-an-exchange-account-on-my-iphone/