It's a secret, but for some time there's been a Google Calendar with Contacts birthdays and events.
It's in Other Calendars:Add:Browse Interesting Calendars:More:Contacts birthdays and events.
No, I don't know what "events" means.
The Contacts Birthdays and Events is public by default. So if anyone knows the URL, they can add it. So you can share it with your spouse and vice-versa, in which case you'll want to give each distinct names.
Obviously it has only those birthdays you've added to Contacts information. This calendar will be more useful if I'm ever able to integrate my OS X/MobileMe address book with my Google Contacts (currently the data models are too dissimilar). So, for the moment, it's more curious than useful.
These published calendars can add up. With this one I have 20 calendars available to my account, though I display only about 5-6 at a time. The calendars I display vary by season.--
My Google Reader Shared items (feed)