There is no fee at the moment for cancellation. The $0 down offer to sign up has been extended until the end of this month, and is likely to be extended again. I base that statement on experience, and not on any special knowledge.
When the DVR is available, you will be subject to whatever sign-up deal prevails at that time, but so will everyone else who has been waiting for a DVR. That will be the time when Voom tries to get the users who have had a DVR with their present service, and you'll have to guess what the sign-up deal will be at that time.