I have a new Samsung F8500 plasma (HDMI v1.4) and an older Onkyo receiver (HDMI 1.3). If I plug all of my sources into the TV, then run an HDMI cable from the ARC port on the TV to an HDMI input on the receiver, will I be able to get audio from the sources connected to the TV?
As long as your Onkyo has an HDMI port labeled for ARC. Not all receivers are ARC capable but you should be able to determine that by reading the manual. Keep in mind that the only advantage of ARC is the elimination of an optical cable. ARC is limited to 5.1, just like optical.