I had a Retina MacBook Pro with Windows 8 already installed. I can boot from it without problems, and all the drivers Apple provided for Windows 7 works perfectly for me except one thing.
I am not able to use Target Display Mode with a Thunderbolt iMac. I understand that Boot Camp doesn't currently support Windows 8, but I am hoping someone has encountered this problem and solved it.
Also, I am able to connect to an HDMI or Mini Display Port-equipped display under Windows, and Target Display Mode works under OS X.