I did notice this morning that the wifi icon in the system tray remained greyed even though I was connected to the internet. I checked my wife's Macbook Air to see if the same was happening there but it wasn't.
I then discovered your post and checked to see what happened after my macbook went to sleep.
Reconnection to my wifi worked okay and I am glad to see that the wifi icon is now black!
Not much help to you I'm afraid.
Had this issue after migrating from a 13" 2012 MBA to a 2013 13" rMBP - it really was the only issue that I was having after cloning the data over the Thunderbolt port. But it's also a very annoying issue everytime you open the lid from sleep.
I removed all my saved networks from the network options, reset the SMC and PRAM, now it's working fine.