For Mountain Lion (Mac OS X 10.8) users
For Snow Leopard (Mac OS X 10.6) and Lion (Mac OS X 10.7) users
Delegate calendars will work when you first add them, but then stop working after a reboot. Currently there is no way for us to resolve this on our end. This is a known bug in iCal's API (bug id 5635463), which is the way we communicate with iCal. This also affects all other applications using the API. As of Mac OS 10.7.3 this is yet to be resolved.
For Google Calendar, there are two way to get around this though. The first way is the easiest, but will only provide read-access. The trick is to add these calendars as subscriptions to iCal, and simply hide them in iCal. For more information on how to do that, please see this article.
The second way is a bit more complex, and we wouldn't recommend this for non-technical users. This way will however provide read-write access to the calendar. The instructions for how to accomplish this can be found here.