Installer came out on Saturday the 10th for a new install (previously had Dish) with 3 HR24-100's. The installer said I HAD to have the DECA device is I wanted MVR and/or VOD. I questioned it, but ended up paying the $25 fee and the installer hooked it all up. All is good.
Sunday evening I get bored and removed the DECA device, plugged a Cat5e into each of the 3 receivers (all connected back to my network switch), reset each receiver, and re-ran the network setup.
MVR works perfectly, as well as VOD (sort of, there's an existing problem with my account that tech support is working on). I have successfully been able to download and HGTV VOD, so I can confirm VOD is working without the DECA. I have also successfully scheduled DVR recording via my PC at work and from my iPhone as well. So yes, they are all connected to the internet.
Last night I called D* and requested a refund on my $25 for the un-necessary DECA device. After being transferred to the equipment department, and after the phone tech talked to her supervisor, I was refunded the $25 without any further issue. The tech said that the HR24 has "an internal DECA". I have not tested that "theory" yet (only one receiver of the three plugged into the network), as all 3 are currently networked via Cat5e cables.
Now, if I can only get DirecTv's "system" to authorize VOD service for an HR24-100 (HR24-500s can, -100s can't for some reason) things will be 100%.