I have the same issue. Sometimes even when i pick up the phone, if i look fast enough i see that the Wi-Fi bars weren't even there! Thats only 1 out of 5 times though. The other time its usually just grey bars. Sometimes it actually will click back and they'll flash to blue and grey 2-3 times and then go solid blue. Then sometimes like you, they'll stay grey. I've tried everything too...reverting to stock, Different kernels, ROMs, routers, static IP, 2.4 or 5ghz, full auto mode. The only fix that makes it a little more reliable, is using Wi-Fi PM Fast. But even that, is not reliable. I'd say its about 50% reliable then, for a huge sacrfice in battery life. For me i've noticed this started to happen with 4.2. Before that i never had a problem. All other devices though no problem. My Nexus 7 is just fine. But the galaxy nexus, no matter what router or where i am, the Wi-Fi is totally unreliable. For me, it happens within 2 minutes that it will disconnect most times. Then 1 in 10 times it'll be fine for 10 hours.
In all honesty, i've given up. Just come to deal with it. Use mobile networks when i'm not needing to download. I can download files and stream music just fine too. If there is traffic going through, it never disconnects. But when its just sitting there without traffic, it disconnects, or, loses connection to google servers i should say.