I recently did exactly this when I dumped Bell Satellite, the kids had a couple of shows they had been watching and I called to cancel my service. I disconnected my 5900 PVR and moved it into the spare room and connected it to the TV that's it, no phone line, no dish and worked fine, the programs are on the hard drive so until you erase them you can watch them.
Just as an FYI, some cable PVRs will not boot without being connected to cable after a certain period of time - about a month based on feedback on this forum. I'm not sure if a similar restriction may not hit a BTV PVR at some time - ie, without activation they will not function?
The only restriction I am aware of with a Bell Receiver is that for the OTA Tuner to function, the Dual Tuner HD PVR must have Signal from at least 91 degree's into BOTH Sat Tuners, otherwise the OTA Tuner will not work.
No idea why they would build the Receiver like this.