Received my iPhone 5 two weeks ago. Starting configuring sync accounts and forgot how to set up shared Google calendar in iOS. Did some research and here is the clue...
Set up your account as a microsoft exchange account.
- Go to Settings -> mail,contacts,calenders -> add account -> microsoft exchange.
- In the Email field, enter your full gmail address
- Leave Domain blank
- In the Username field, enter full gmail address
- Enter password
- Enter m.google.com in the Server field
Now, the important part. This step allows you to choose what calendar(s) you want to see.