Exactly, the receiver is the source and it's OUPUT is connected to the TV's INPUT. I've never seen a TV that could send any signal out of it's HDMI input.
Do you have the HDMI Monitor setting on your Onkyo set to on? If so, all your sources should be upconverted and output through the HDMI out (as well as the onscreen setup menu for the receiver), see pages 47-48 in the manual. Once you get the new HD DTV box, you'll still be in a similar situation as now. You'll have to connect an HDMI out on the DTV box to a second HDMI input on the TV, and still have an optical cable going from the DTV to the Onkyo. Unless you ditch the DVD player and use the PS3 for your DVD/Blu-ray.