802.11 ac will do nothing for your internet speed unless your isp offers something like 500mbit/s speeds or you transfer or stream files.
There are three possibilities.
1. The retina has three wires connected to the wireless card (from what I can see). This is good. In the past Apple has been nice remember g to n in the old MacBooks? Apple will make the card!
2. The retina is capable but Apple says no, a third party eventually releases a internal card!
3. The retina is not capable because of the antennas and you need to buy a usb 802.11ac device.
I have a 15 retina and hope that apple made this work for ac. However its really not like my 802.11n with time capsule is slow in anyway. Its really only for file transfers. I have a very fast internet speed compared to most 100mbit/s download. My 802.11n is more than capable to get the most out of the speed I have. So remember unless you transfer a lot of files over a network or have a 1gbit/s internet speed which I doubt, this upgrade will do little to nothing for you in speed increase on the network.