Turn off the PC.
Turn off the HDTV.
Connect the PC to the TV using the HDMI cable.
Power on the PC.
Power on the HDTV.
Change the HDTV input to the HDMI input the PC is connected to.
This should work. However, ATI (stupidly in my opinion) automatically sets any display connected via HDMI as the primary display. If there is an issue with the display on your HDMI connected HDTV, there is no easy way of correcting it.
Note: You may need to go into AMD Vision Engine Control Center and set up multiple displays in extended mode.