I also had this problem on Retina running Mountain Lion.
After waking from sleep, WiFi doesn't automatically reconnect although it connects just fine when I manually select the network from the list.
I found a working fix here, kudos to Morris for posting.
For convenience, I reproduce his post here:
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.