I'm having a similar issue with a HD 5830, and a Panasonic TH-P60S60D TV. Monitor is connected to DVI, and it works fine, but HDFI stys blank with no signal from card while in Windows. During boot-up, there is display on both screen. Once in Windows 8 x64, no display on TV.
Did any of y'all sort this out? Do you know of a solution?