I took it over to my old place and right after it synced, all the lights started flashing and it restarted, I logged in and sure enough RCN had already pushed their firmware to it (bye bye cox!), I didn't even have to call them. I'll try to activate it again tonight with RCN firmware and will report back with my findings.
Yeah, some providers authorize firmware updates on any connected device, others only on authorized. Comcast in your area probably only on authorized because even that RCN firmware will go bye bye once activated.
Your final FW should be: d3000-v302r125532-120517a-L2VPN-CMCST.bin
That said, the online activation will probably likely fail still as it will be an unknown modem resulting in you calling or getting level 2 support to add it. You can play this two ways, play dumb saying it is an 3008 which is the official supported model (though they are the same basically) or be honest and they may still help as they can override the model info. Really depends who you get :P