(someone was kind enough to remind me to update this

)
At the current date (December 25th, 2003), ReplayTV service is $12.95/month or $299 for "lifetime" service. Activation is *per box*, and belongs to the box. This means that if you sell your old box, the activation goes with the box and any newly bought box needs new activation.
In summary, Replay's official product line:
2XXX or 3XXX models: lifetime activation included on all boxes with these model numbers.
4XXX: lifetime activation included on all boxes with these model numbers.
45XX: Requires paid lifetime or monthly activation
5XXX (unless bought at Radio Shack for $499 and up): Requires paid lifetime or monthly activation.
5XXX (bought at Radio Shack for $499 and up): Had a 3-year activation at sale, now has lifetime activation.
55XX (If bought for $499 and up) was sold with 3-year activation at sale, now has lifetime activation.
55XX: (If bought for $149 [40], $299 [80], or $449 [5160]): Requires paid lifetime or monthly activation
Users who registered their monthly activation at earlier price levels so far have continued to pay their original price. Any changes in service (like changing the billing person or converting to lifetime) will convert the unit to the new pricing level.