Ho anche avuto questo problema su Retina che esegue Mountain Lion.
Dopo il risveglio dal sonno, WiFi non si riconnette automaticamente anche se si connette bene quando seleziono manualmente la rete dall'elenco.
Ho trovato una correzione operativa qui , i miei complimenti a Morris per la pubblicazione.
Per comodità, riproduco il suo post qui:
The first step is to create a new 'Location' under 'Network Preferences' and use that as default. I have deleted all old locations, don't know if that is necessary though.
The second step is to follow Apple's advice in Troubleshooting Wi-Fi issues in OS X Lion and Mac OS X v10.6.
Expand the Solution section under Symptom: The network connection drops unexpectedly and follow the steps listed under Symptom: After restarting or waking from sleep, my computer might not connect to the Internet.
Just make sure you write down all the passwords for the networks you want to keep as deleting the keys in keychain will permanently remove them.