I'm currently trying to subscribe to my Google Contact's Birthday calendar from Hotmail, so far without success.
I've used the ical link from the calendar settings in Google to create the subscription and it appears to create successfully.
Going back into the calendar settings in Hotmail the same error is repeated five times:-
Errors: This calendar wasn't updated because of a problem with the publisher's file. We'll try updating it again later.
A search on that phrase hasn't turned up anything useful to date.