OS X 10.8.3, Alfred v2.0.1 (173), Cobook 1.2.2
Cobook is set as my default contacts app, however Alfred won't open contacts with that app (nothing happens when I select the contact in the Alfred results).
I get the same thing on two different Macs (running same versions of the apps). This did work with Alfred v1. I can confirm that the contacts do open in Alfred v2. I have not tried using Apple's Address Book as the default contacts app.