Understand that this might all change with iCloud. We'll just have to wait and see.
Until then, I was still missing birthdays because they never did synch from my Address Book and if I updated the Google contact, they didn't show up in my calendar. Until I found out about this handy little tip.
In Google Calendar, under the "Other Calendars" section, click Add and then select "Browse Interesting Calendars".
You will be shown a list of "interesting" calendars. Select the More tab and the first one in that list is called "Contacts' birthdays and events". Subscribe to that one and you're good to go!