UPDATE: 9:15 am 1/14/2013. The Google device developer assigned to the Nexus 4 wifi issue just posted in the code.google.com thread dedicated to this problem:
"Regarding nexus 4: Qualcomm driver on nexus 4 has a known issue of not supporting ARP offload while filtering out broadcast packets. This causes issue with connectivity when device is in suspend. Fix planned for next update."
So we now learn that for all this time, this was a known problem with the wifi driver used in the Nexus 4. Yet Google Device support blithely continued to RMA Nexus 4 phones for people who called in complaining about the wifi going to sleep when the screen was blanked.
Oh, and the Google Device Support Team never mentioned that this behavior was due to a known issue. I wonder if being incompetent violates Google's official corporate policy?