This is apparently rare since the folks at customer support had never seen it happen until I came along.
My unit, which I just bought used, was saying something like "There is a problem with your account. You need to call customer support before you can use your ReplayTV device."
I called customer support, and they were stumped. My unit has a lifetime activation, and after forcing a net-connect and resetting to factory defaults and resetting my network connection, System Information still said "deactivated". It wasn't recognizing my lifetime activation.
After 45 minutes on the phone, the representative, who was great (I think they're all great), "relapped" my activation because he had no other ideas. That just meant that he reset my lifetime activation. He said it would take an hour or two to take effect, and it did. But then it worked great!
So that's what to ask customer service to do if you know you have a lifetime activation but your machine keeps saying "deactivated".
Hope that helps someone sometime!