I am seeing the something simlar. When I add a server-based calendar to the 10.8 Calendar client, the Push option is not immediately avaialble. But, it shows up after the connection is made (and authenticated) to the server, or after quitting and re-launching Calendar.app. This setup is using a 10.7.5 Server.
However, once you choose the Push option, the Push notifications do not seem to be working at all - but this is only true for 10.8.2 clients, Push notifications work correctly on 10.7.5 clients. When I set the 10.8 clients to automatically refresh every 5 minutes, thing appear to work normally. It is only Push that is failing.
Very odd behavior. Might be a bug in the 10.8 Calendar app.