I've setup Visual Studio 2013 RC and a Office 365 developer site. I can create an app using Nappa and open it in VS from there. But if I create a Sharepoint hosted app project in VS and point it to the url of my developersite it get stuck on "communicating with sharepoint" for a long while and then asks if I want to set the project in "offline mode", obviously I click "no" and then visual studio hangs.
The answer is not obvious. You need to prefix the url of the site with httpS:// and not http://.