Between rc2 and rc3, the assignment of the "protocol" variable in the completeRequest function (line 7090) has flipped the order for where it tries to find a protocol. Previously (in rc2 and prior), it was first checking the actual request for the protocol before trying to get it from locationProtocol, now in RC3 it reads:

var protocol = locationProtocol || urlResolve(url).protocol;

This happened to break compatibility for me in PhoneGap/Cordova - the code making the call is stored locally, with the file:// protocol, while the site I'm making requests to is on an http:// protocol. (NOTE: I know this is technically an XSS - PhoneGap has its own domain whitelisting system for preventing this from being a security hole)