I found this link on how to use shared calenders from google on the iphone. The description is in Swedish so for people not fluent in this beatiful language here is a short summary:
Make sure you have the calendars you want in google. Take one calendar at a time and do the following:
In google calendar click on settings->Calendars->The Calender you want to add->Calendar Address->ICAL
You will see an address that looks something like:”http://www.google.com/calendar/ical/tester%40gmail.com/public/basic.ics”
In the iPhone’s Settings -> Mail, contacts, Calendars > Add account … > Other > Add CalDAV-account > and write the following in the fields:
Server = http://www.google.com
User name = firstname.lastname@example.org
Password = xxxxxxxx
Description = (E.g. Name of calendar)
Click next -> “Advanced settings” and paste the address you got from google above between dav/ -and- /user, i.e. dav/swedish__sv%40holiday.calendar.google.com/user. Let the text before “dav/” be unchanged.
Save and enter the calendar app to see that you have your first google calendar there. Repeat for the rest of your, your wifes, kids etc. calendars.
The above also saves the Exchange ActiveSync account for use with an exchange server.