Looks to be due to the addition of the else if(window.XMLHttpRequest) in invokeTransport. SC.Event.add doesn't actually add the readyStateChange handler to the raw request.
Simple fix: remove the else if. Not sure if this will break things though.
Code in question is here:
on line 552. Looks like commit bc0b8fc (adding support for XHR2) added the offending else if; I guess IE8 has window.XMLHttpRequest but SC.Event is failing to add the handler there. Anyone know what IE8 event handler weirdness could cause this?
@rcatalano reports that the problem is that in IE8, the rawRequest object has neither addEventListener nor attachEvent methods, so SC.Event.add will fail. Sounds like the solution is to check for (rawRequest.addEventListener || rawRequest.attachEvent) on line 552 and fail over to the below "else" if neither are present.
Any chance we could get a pull request please?
Fixed in 1-9-stable and master branches.