First of all, as far as I know, DOD (Directv on Demand) has always been a FREE service from directv.
Alright, So, Directv has just pushed through an update that DOD is now available to ALL HD-DVRs. You don't even have to be connected to the internet.
The fine print: If you are connected to a router via Ethernet cable to your receiver, you get more content faster. The reason why is that you have unlimited access to the internet through your broadband whereas Directv has limited bandwidth through the satellite.
What does this mean? If you do not have your receiver hooked up to the internet, you can still get DOD programing but it only downloads once the bandwidth is available. This could take up to 7 days to START downloading.
So, if DOD interests you, I would suggest connecting your receiver (the directv box) to you router via Ethernet cable. Just keep in mind that you have to have an HD-DVR to take advantage of it.